It's a good idea in principle but I think we're on a slippery slope here...
In my game development experience it is much better to tweak existing units to fill multiple roles better than to just keep adding more and more types. I understand your argument about fighters needing to be in the middle ground to prevent bombers from being completely useless, but there is always more than one solution to this sort of problem.
Were I on the dev team, I'd find it. But my time would be wasted coming up with one as it is, the odds of a dev noticing is just too low, and it's too late tonight to start arguing my point properly.
Here's the fact of the matter: when you add too many units in too many roles/counter-roles, here's what happens. You build a bit of everything. You tweak a little here or there depending on your experience, but you just build a bit of everything because you don't know what each of your enemies might throw at you. There's another word for this, I call it game-cancer. The unit count grows and grows but in fact you could be building just one type of unit and you would have almost the same gameplay, just less pretty.
You'll build 20 fighters, 20 bombers, and 20 interceptors. You know what? You might as well have just built 20 "multi-role" fighters because the group as a mass is necessary in order to be effective - you said so yourself. The whole concept of this ancient rock-paper-scissors gameplay design reeks of the early 1990s. One thing is overpowered, well let's make a unit to counter it. Well now this other thing's overpowered, let's make a unit to counter it. But now the counters are overpowered, let's nerf them. It baffles me that this unit-counterunit-countercounterunit gameplay is still around. Unit-specific damage, R-P-S... We're still freaking playing Age of Empires 2, just in space. Rather than fix the root cause of the problem, you just band-aid it, and almost always in a way that just makes another problem.
Can't defend planets adequately, units run right past defenses.
So add Starbases!
Starbases are too powerful.
Add anti-starbase ships!
Anti-starbase ships are too powerful.
Now they're more trouble than they're worth.
Buff them, but make a unit REALLY GOOD at killing them. Let's say... fighters!
Fighters are overpowered.
Buff anti-fighter units!
Fighters get smeared across flak ships' windshields.
Nerf flak ships!
Bombers are too powerful.
Nobody builds bombers anymore, just fighter swarms.
Errr... Make an anti-fighter fighter?
All of this, all of this, to fix "units run right past defenses." Come July or August, maybe then I'll make a mod or something, I'll be busy until then with work.
Hey, if you want to see how a game runs without massive R-P-S, unit specific damage, and all this nonsense, get yourself a copy of Total Annihilation and I'll give you a download like for my mod I made years and years ago. The game balance is still completely unrivalled, except for the fork projects that started after I didn't have time to work on it anymore and was generally satisfied enough to call it "a finished product".