True. But why does the function of a sniper have to be to kill? In RL, it is to kill because we don't have the protection or the healing, and the sniper has the edge in technology. 50 caliber silenced sniper rifle anyone?By LokiDR
Finally, there is little chance of one shot-one kill at mid to high levels. Unless you have lots of sneak attack and can do it at long range. Magical healing being what it is, and how much HPs stack up. Even wizards HP are helped out by endurance & false life, AC by mage armor & items.
So what has the role of the sniper become? To harry. To confuse. In a tense confrontation with NPCs where diplomacy on the party's part is needed a shot from the sniper attacking the other NPCs would create havoc; likely the NPC group would distrust the party and possibly attack, thinking the shot to have come from them.
The sniper doesn't have to kill. He just makes life tough. If the party is traveling, he can singlehandedly prevent any of the casters from regaining their spells. All he has to do is wait. An arrow in the back of a sleeping wizard (-4 to hit for being prone, but a loss of DEX and probably spells like mage armor) is easily done. And that arrow means the wiz needs to restart the 8 hour resting cycle. Sucks to be him.
Of course, that changes once Mord's Mansion comes into play, but the party can still be harried by a sniper.
Rules question: can a sniper perform a ranged touch attack on a target? All he wants to do his hit him somewhere, and not necessairly do damage.
If he can, would the target be considered to be in combat? Would that disturb a caster enough to stop spell memorization?
If it does, when I am an evil overlord, I will have my Leigons of Terror include a special branch of service tailored to distrubing the rest of casters.

Another idea for snipers! Have them carry arrows tipped with the pyrotechnics spell. Either the smoke or the flash effect would greatly hinder the party if combat ensued, or even if combat didn't. Then the party would stop using fires to light their campsite.
If the party used magical means of lighting: arrows tipped with vials of alchemist's fire; follow up with a pyrotechnics arrow. Snipers work in teams, right? Have the pyro-arrow sniper ready an action to shoot at the alchemist's fire. They both go in the suprise round, and both are able to duck out of sight. He he he.