In realistic space combat where large ships are engaging each other over millions of miles of vaccuum with lasers and missiles. Would fighters actually be any use?
In surface naval combat they are useful for pojecting power over a greater radius and engaging targets over the horizon, being much faster and more elevated than the ship carrying them. But given that there's no horizon in space and also the vast distances involved, I wonder if fighters would actually be any use - certainly not the X-Wing or Star Fury type. Perhaps a sort of small, fast gunboat might be better, being able to close the distance between itself and the target more quickly, increasing accuracy?
It depends a lot what you mean by "fighters".
The fighters that have flown off aircraft carriers since WWII have been useful for protecting ships from bombers and torpedo-planes, and for projecting air-power over land from a mobile airbase. Bombers and torpedo-planes have been useful at sea because they travel in a different medium from the ships using a different propulsive technology that gives them performance characteristic that are better than ships in one way (faster) but worse in another (much less endurance: hours instead of months). The trade-off made it worth building both ship and planes. The threat to ships &c. from bombers and the need to escort ones own bombers made it worth carrying fighters.
That a situation much like that will arise in space seems to me very unlikely. The main reason is that there is only one medium, not two. If there is a short-range potent weapon that you can mount on a small vehicle, you'll get something more like a torpedo-boat than a bomber. And with as you say no horizon and no stealth in space (at least for a high-performance vehicle) I expect that torpedo-boats would be dog-meat for long-range defensive fire.
Beam ranges of millions of miles, even through vacuum, would require either enormous objective apetures (mirrors or lenses) or such short wavelengths that we don't actually have anything that will reflect or focus them in a suitable way. But tens of thousands of kilometres, even 50,000–100,000 seem possible. And any realistic propulsion system seem unlikely to be able to get a small craft to cross such a distance in least than about an hour — probably much longer. That is a hell of a long time to withstand any sort of defensive fire. If it can be done, then the performance burden of carrying a human crew seems prohibitive, a computer is a thousand kilos less massive that a crewman and his life support, and cheaper, and probably better. And a disposable missile bus only needs enough delta-vee to close with the target and make a fast pass: a manned bomber needs that, and as much again to stop, and enough to return to the carrier after the battle. I very much doubt that "torpedo-bombers", if there are any, will be manned.
Whether fighters might be useful to defend ships against such attack drones is not as clear, but I doubt that too. You just can't get a high-performance sensor onto a small ship, and there seems to be a huge advantage to having a large objective mirror/lens in any weapon. And the attacking "bombers" will be closing for a fast pass at speeds built up over tens of minutes to hours of acceleration, not limited by airspeed as airborne bombers are, so closing in to dogfight with them seems out of the question. The only "fighters" I can see working are a sort of missile that manoeuvres a sort of claymore mine in to the path of incoming missile busses and fills it with a cloud of shrapnel. I can't see any advantage in manning them.
Frankly, I suspect that the current joint strike fighter will be the last new manned fighter built for use in atmosphere, and that by the time we are fighting in space it will be all long-range plinking with beam weapons or wholly dominated by drones. Or both.