And now that i have Responded to Dr. B I am going to make a list of reasons that Space fighters would still be used.
1. They are cheap.
Strike craft are going to cost a fraction of the cost of even the smallest frigates. While a frigate may be overall more cost effective it doesn't actualy mean anything if you can't afford the entire frigate. Small groups such as coprate security forces and police forces can afford to field more fighters than they could frigates and their fighters can cover a greater areas.
Along with the low cost is the fact that you can have large amounts of fighters. Small fighters are a sufficent force to defend things like small mining facilities and when a group has numerous things to defend its impractical to have capital ships at all of them. This point is even more important in times of peace when a full fledge attack is not going to happen, but you want to defend against minor pirates and such.
2. Fighters are good at killing other small craft.
Fighters make for an excelent counter agaisnt other fighters. As stated in Point 1 small groups are likely to almost exclusivly use fighters making having more fighters a simple counter. Fighters are also good at attacking dropships, merchant ships, and other reletivly unarmed targets.
Outside of large battles fighters and move around and pick off enemy ships as they go about their buisness (think Subs in WW2). You wouldn't want to send a cruiser to attack an enemy transport thats close to their defensive lines, Renforce ments might arive and destroy your more valuable cruiser. Even if they don't you still moved a much more usefull ship out of formation and weakend your lines, annd the cruiser would be over kill for an unarmed transport. However moving a squad of fighters wont really weaken your lines and if your fighters are destroyed their lost cost makes that mostly unimportant. And you can always add more fighters until there is enough to destroy the target without overkilling it.
Inside of Battle fighters can move around and attack the enemies troop transports and other soft targets without reducing the strenght of your main fleet. if the enemy wants to keep those ships alive he will need to move other ships to defend them or move them closer to the actual battle. Assuming the enemy does the same thing then you will need to have fighters on hand to help counter his fighters.
3. Fighters are still ships and still need to be shot.
While fighters are nowhere near as powerful as larger ships, they still have weapons and must be dealt with. The enemy can't just ignore them or they will get severly damaged. Point defense weapons, while small will still damage larger ships, even capital ships. Wile the enemies point defense are keeping your fighters at bay, they wont be shooting at your ships. While the fighters are occupying the point defense they will also still be cuasing some damage on the enemy.
4. Fighters are the only ships that can effectivly enter a planet's atmosphere.
Fighters will be the only things that can escort dropships onto a planet. without fighters to protect them the dropships will get destroyed by anti-air fire and enemy ground bassed fighters. After they escort the dropships in they can even aid the ground forces with air cover.
And since we can't discount the possibilty of planet bassed weapons that can shoot ships in orbit, fighters are the only things that can take this weapons out with out taking heavy casualties, and without bombarding the surfase of the planet and causing massive collateral damage.
5. Fighters (and other small ships) are the only things that will be able to move effectivly inside of a plenets gravity well. (probably)
Bringing capital ships right next to a planet is probably not going to work. Unless some kind of tech reduces the affects of gravity large ships are going to have a really hard time moveing around next to a planet.
6. Fighters (and other small ships) are the only thing that you would want anywhere near a planet.
When a fighter gets shot, assuming it isn't completly destroyed, its going to drift into the plenet's gravity and get pulled in. it probably wont make it past the atmosphere but if it does it will amount to nothing more than a small meteorite hitting the planet. When the Super battleship (think a Kol) gets shot its going to do the same thing. Except its not going to burn up in the atmosphere, its going to land and take out a continent atleast. Then its engines and any war heads are going to go off and thats going to make matters worse. If your defending a planet your not going to want your ships or the enemies anywhere near it. IF your trying to conqure the planet the point will be lost if you accidentaly make a crator the size of australia on it.
Not only are falling dead ships a problem but you also have to deal with missed shots. when a fighter misses and the shot goes wild nothing is going to happen. even if it does hit the planet it wont have any greater effect than if the fighter itself was on the planet when it shot the attack. When a batteship shoots a slug from a mass driver thats the size of my house its going to mimic the same effect that a planatary bombardment would have had. If your goal isn't to destroy the planet your going to want to avoid accidentaly firing huge missile, energy weapons or kinetic weapons at it, or accidentaly crashing a ship on it
7. Fighters can be built easily, quickly, and in huge numbers.
First Speed. Fighters manialy due to their size can be build very quickly. In times of war when your manufacturing capacity is maxed out it isn't unreasonable to assume that a medium grade fighter could be built in a few hours. A empire that makes use of fighter craft could drasticaly increase the size of its fighter forces in very short amounts of time. as the war wears on it can easily recover its losses of its fighter squads where as the building of frigate, crusiers, and battle ships will take much longer.
The ease of which they can be build and the amount kind of go hand in hand. The needed facilites to make a fighter could proabably fit inside of a small autobody shop. New Jersey alone pobably has enough facilities that could be converted to fighter production with in a week that within another day it could produce 10,000 fighters.
Your frigates and crusiers are going to need orbital facilities to atleast ground based ones with some form of transportation aid to help them get of planet. technicaly the frigates and smaller crusiers, if properly prepared, could be built on planet and transported into space in large numbers and releativly fast, but not even coming close to comparing to the rates fighters could. The facilities to build the frigates and cruisers are also going to be more costly and harder to make and the parts are going to be more costly and harder to find.
While the frigates and crusiers can still be made relitvly quickly and with some degree of ease and in decent numbers the massive capital shis can not. Capital ships are going to take years to build in times of peace and in times of war could be rushed into service in a matter of months, a far cry from the few hours fighters can be built in. Capital ships will require an orbital shipyard to be built; the shipyard is going to be just as costly and difficult to build, if not more, than the ships it builds. Most shipyards are only going to be able to build a few ships at once and not all planets will have a shipyard at all. Not only that but becuase of their stratagic value shipyards will be a prime target for enemies. and even more so if there is a ship in construction inside of it.
8. Fighters are less demanding on personel resources.
A fighter is probably not going to have more than 2 people in it. That paired with their low tactical value they will be veiwed as almost disposable. new pilots can be trained and combat ready in as little as 1-2 weeks. You could easily have an entire division of fighters trained with in a month. In comparision the Cobalt light frigate has a crew of 350, that includes technicians, communications officers, damage control teams, weapons control teams, navigators, and the bridge crew. The Kol Class carries 2500 people. Not only do these ships need various experts in different fields but it also needs a good bridge crew to command the entire crew. Captians who are skilled enough to command entire ships are usaly pretty rare and a valuable resources.
So thats all i have for now, however those 8 points are pretty solid.