I dunno if you guys noticed, but he is talking about AI only. To say that static defense structures are 'monumentally useless' in multiple AI matches is about the most absurd suggestion I could fathom as a suggestion to improve chances of winning against multiple AI. There's either a misunderstanding about the playstyle here, (static def is indeed poor in multiplayer for obvious reasons) or a gross misunderstanding of how the AI behaves. This isn't WW2, and Patton wasn't talking about programmed intelligence.
I)It's hard to really analyze what you're asking here, because the time frame is important. In the early-game (first 15 min) planet grabbing period, almost all your income should be going toward economic infrastructure and the ships necessary to acquire planets. In some cases, certain planets may be vulnerable to small attacks of 1-2 frigates or a few wayward siege frigs. 1-2 turrets facing the phase lane they'll enter from will turn the majority of these attacks away. Other than that, ai will generally not commit to attacking your colonized planets until there are no neutral planets left for it to acquire. To do so would put it at a disadvantage, because of how long it takes to kill planets. Now is not the time to be dumping your crystal into static def.
When the battle starts moving into the middle game, static defense is going to be more useful. Now, if this is a 1v1, or even a 2v1 depending on your skill level, your money will be better spent investing in a fleet. This is not because static structures are 'useless' in this scenario, but because you are not made vulnerable by playing aggressively. If you take the fight to them, there's no reason to leave credits at home. But if this is a bigger FFA or 1v3+, you are going to want static defense at any planet that is on the front lines. The purpose
here may not be what you expect. Defense structures are not there to save you from fleets, or even to kill ships. They are an incredible deterrent to small-medium ai attacks. You can have the strongest fleet of all the players, but it can only be in one place at a time. If you leave your planets to attack one comp, which is a significant time investment, your accessible worlds will start getting peppered by other enemies. If you don't redirect your fleet, you'll lose 4 worlds while gaining one. In response, your fleets will be pinned to full time defense, and the game will invariably draw out for hours and hours. You need to be capable of keeping enough firepower at home to convince the AI that it won't be able to take your fringe worlds, even while you attack elsewhere. You can do this by either researching a higher and higher fleet cap, which will tank your econ with irreversible upkeep, or use static defenses to boost the numbers. There's a pretty obvious right choice here. Yeah, static def takes funds away that could be used toward fleets, but driving your upkeep up for bad reasons also takes funds away, and leaves your planets in constant need of babysitting. Keep in mind that if a heavier attack comes that your current defense won't actually beat, the existing defense gives you time to construct more turrets. Comps are stupid and don't attack the builders most of the time.
If things make it to a late-game sort of stage, (you're touting fleets comprised mostly of high level cap ships and heavy cruisers) you'll probably
want to flesh out your static defenses to their actual caps. I say probably, because it depends on whether money is an actual concern by then. In the late game for me personally, I generally end up having more money than I can reasonably spend.
Long answer, sorry. This is kind of an important issue in winning vs-AI games, and I think that you would be severely handicapped in big AI games if you stopped building orbital defense as suggested in this thread. 1v1, sure, 4+ player FFA, no.
Research is critical to your military but less so for your economy. To put that another way, you win by fighting not but amassing lots of cash. That being said, the more useful civ techs are the early ones -- terran population increases being the absolute best bang for your buck and they are tier 1. The easiest way to expand your economy is to expand you empire by taking new worlds, which is a function of military, not civ, tech. Remember, never purchase a tech you won't use immediately. And NEVER buy labs you can't immediately use.
Just quoting this, because it's pretty much what I'd say. Trade ports are big, throw down on the passive econ bonuses if you get a good opportunity, but the important things to research are things that give you a new ship, structure, or ability that you will actually use. Don't think you need something researching all the time, that's a big early game trap for new players.
III) Aside from certain capital ship pairings like Dunov+Kol(s) or general passive auras/etc that you want for a fleet, basic ships are generally better. This is especially true in the early game, where capital ships do not actually produce much firepower. This is subject to context, though, and some builds use capital ships more numerously. Explosive Nanites + 3-4 Phase Missile Swarm spamming Desolaters, for example, can be quite effective.
IV)Culture is primarily a late game tool, for various reasons. Reason 1 is the cost. ~900 credits and a hefty resource bill make them prohibitive to use early. The 10% income they generate takes a long time to make back its cost, and you kind of need the logistics slots for other things. However, when money is not so much an issue, and research has gotten deeper, they can be very good for non economic reasons. The TEC version of culture research is probably the best of the three, as the antimatter regen is quite significant to capital ships and support cruisers. They're all good, though. Culture can also suppress the loss of successive planets in a gambit to trade some of your planets for the eradication of one enemy. It is then also easier to get them back when the scenario has played out.
Feel free to PM me for more specific questions/suggestions.