Good grief, I take a week or two off from the forums to kill my burnout and y'all have 20 page discussions on me... And a new expansion, but I'll post that in the relevant section.
I'm not a fan of the fighter because it doesn't fit my playstyle. It is, however, a powerful enough ship. This entire discussion however went from playstyle to fighter discussions... errr, hang on a second, let's drag that back over kicking and screaming, shall we?
What is your playstyle? I personally like to definatively answer a system in one stroke, then leave it gasping for air as I move on. What's good for this? Bombers and whatever the hell protects them. In my case, Missile Frigates. I'll take the cost and speed loss for the overwhelming firepower and range counters. Also, keep your damned fighters off my bombers, I lose them like candy at a children's convention anyway.
So, tossed into that is a bonus ship (or three). I probably ended up with a fighter wanna-be along the way. What does that have to do with the fighter conversation? Not really a damned thing, actually... until you look at my early game. We'll get there. So, in general, my massive fleet-ball with some minor key-binding adjustments for extreme units (snipers/maws/special micro or uses) will walk into a system, plant a flag, and argue with the AI over continued existance. One of us lives through it. I only open Fighter MK II+ if I'm explicitly fighting a polycrystal heavy AI, and then they're invaluable.
However, that's not reasonable early game... or at least wasn't until recent economic improvements for early play. So, let's back this off 'stupid difficulty' for a second and drag this back down to 7/7, neh? The fighters were your permanent younglings. While your main ball of frigs and bombers were out taking care of business, you could go and raid a system with your fighters, get them obliterated because your attention was on the rest of the fleet, and then come back and whatta ya know, they're pretty much rebuilt. They're cheap, semi-effective, and an excellent experimentation and probing tool. Raids are far too expensive to probe with.
The balance of any ship is cost to usage. In late game and against significant wave/reinforcement, the fighter fails. At that point, you're dumping huge volumes of mats (to the tune of thousands/second) into massive starship builds, streaming FRD assaults with younglings (much better at the cheap meatshield dental plan), rebuilding turret walls from waves, and if you're like me, cursing out your metal reserves because you're rebuilding your entire Bomber Flight AGAIN. But that's later.
In the early game my fleet builds at 10 scout/50 fighter/5 bomber/5 frigate/x bonus (depends). Those fighters are invaluable to get the early game going. THEY ARE WEAK, and they should be, for their pricetag. They don't have much use for me late game because they're utterly outshone lategame. I personally think their MK II/III K costs should reflect that, but that's a completely different balance point, as the point of a K change would be to allow for early upgrades when the economy is a stringent issue and you haven't had a chance to raid deeply and get a huge econ going.
However, you have to consider the golden hours and the power of the fighter's cost there before it's dismissed. Full fleet is usually busy tackling serious threats, like local MK III/IV worlds and digging out Tachyon posts so your scouts and get to work. You need that disposable fleet to work.
So, to the original question, what's my playstyle:
Heavy fleet on heavy targets with light ships doing side work while my Raid Starships get built. Eventually, my fighters become cheap useless meatshields in the fleet as more effective bonus ships are opened up for whatever particular purpose I'd been using fighters earlier. At that point, the fleet ball goes for defending freed ships while my Raid Starships free up enemy ships and chew them up as they escape the Raid assaults.