The time it would take a weapon firing at the speed of light to travel from one planet to another would actually leave you a fair bit of time to react, assuming everyone is on their guard.
And if we're limiting the range of these weapons based on targeting sensors, it's really up to the sensors themselves. If they can lock a target that far, then why fight with vessels rather than orbital platforms and colony ships?
The biggest reason to fight as close as the same grav well is to make weapons like bullets and missiles useful. Lasers could be useful from afar, but that depends on our ability to maintain cohesion at a distance. Like with most sci-fis, a big problem is putting it in the year 2XXX with no idea how tech will work out come that year.
As for strikecraft, they can still be useful. Not only can they avoid large turn radii by simply turning on the spot, then compensating, but if engine technology progresses far enough to make a missile operate efficiently, then a fighter too could develop the speed and maneuverability to provide a fair opponent. Of course, beyond a certain point you either have to have all fighters unmanned OR develop some kick-ass inertial dampening system.
Now that I think about it, the best reason to use a fighter is that if designed properly, it's radar cross section could be crazy small, allowing it to slip in without notice by many automated weapons and deliver a payload. Of course, to counter visual identification or red-shift detection, it would have to absorb light AND have no external lights. That means precautions would have to be taken for safe launching and landing.