^^
For ranged attacks, the factor is the level of specialization. By the time you're so specialized that your missile attacks have a 10 point gap in accuracy, you should probably have the funds for SOME source of flight, even if it only works for a few rounds each day.
A barbarian with a starting 18 str, 12 dex has a 5 point gap already at level 1. More if you pick a race with a str bonus (like orc) and/or you consider charge. But yeah, I see your point, most other melees would have a 3-4 point difference at low level. The main problem isn't only accuracy, but also damage. In melee you're probably operating around 2d6+1.5Str+ other bonuses (PA for a 2 to 1 return, and even most low level ToB strikes provide decent damage bonuses). At range, you're down to 1d8+str, and most likely nothing more. Take the aforementioned 18 str 12 dex character. If he's a barbarian with power attack, he attacks for 2d6+11 with rage and PA (avg 19), his ranged damage, even assuming he has a +6 mighty longbow is 1d8+6 (avg 10.5 damage). If he's a Warblade that somehow got bow prof. he could be using Steel Wind for 4d6+12 (avg 28 damage) or Wolf Fang Strike with armor spikes for 3d6+8 (avg 18.5) while with ranged he's down to 1d8+4 (avg 8.5). This is all anecdotal evidence OFC, but as you grow in level the gap increases (exponentially in case of charger builds). Any situation in which you do maybe 20-30% of your regular damage output and your primary role is a damage dealer is a situation in which I'd call that char useless.
As for flight in general, until the levels where you CAN afford persistent flight, most fliers are melee or short ranged(breath weapons, SLAs with range < 60ft etc), which means they can either engage, or flee, in either case you can count it as a win for the PCs if they flee anyway.
A short skim through the SRD of flying monsters of CR<5 and how I think they compare with a fighter that drank a potion of Fly:
Air elementals (small, med, large): Melee only, but with flyby attack and 100 ft. speed. Any attempt of chasing them probably results in being strafed to death. Can be beaten/driven off by playing smart (not chasing him since he has to come in melee if he wants to do damage).
Allip: only 30 ft. speed, but incorporeal, drains wisdom and fighter must beat Will DC 16 not to auto-lose
Lantern Archon:60 ft. speed, 2d6 ranged touch attacks with 30 ft. range. and only 4 hp. My money's on the fighter despite DR 10 if the archon doesn't flee (which he can do).
Arrowhawk (juvenile and adult): 60 ft. fly speed, 50 ft ranged attacks. Can't attack without getting stuck in charge range. Money on the fighter if he doesn't flee.
Dragons (several): Much higher speed than flying fighter (100-150 ft.). Can Flyby attack to death with breath weapon if built this way. No way I'm giving flying fighter any chance on this one.
Darkmantle: Low fly speed, melee only, low grapple check. Fighter beats it with ease.
Imp/Quasit: 50 ft. speed, invisiblity at will. As written they can play hide and seek with fighter all day long. Now give them a bow (which they can use being outsiders) and the situation radically changes.
Giant eagle: 80 ft. fly speed, but large and with 3 decent attacks at CR 3 odds are he's putting the fighter in trouble in melee. still, point for the fighter in the current discussion. Same goes for Griffon and hyppogriff.
And this is where I got bored
Surprisingly I am now convinced that a low level fighter isn't that helpless vs low-level flying opponents. Yes, there are some that render him useless, but TBH he can stand up (mainly drive off) to a surprising number of them.
