Phal, the sniper can be one of the most dangerous classes in the game, i merely say try playing a good team so that they can show you why

all theories will go out of the window.
Yes I know :'( *sigh*
I know Snipers can kill a lot if theyre half decent etc blah blah BUT
BUT!!
They are too easily circumvented ( go another route to avoid them )
They die easily
They are crap close / medium range
They have a very narrow field of vision when zoomed in.
They get sucked into Sniper duels and then you effectively lose a member from each team
They have a slow Firing rate
They bring no other useful features/perks to the rest of the team.
They cant even jump on a point to defend it when it gets rushed since theyre usually too damn far away.
They cant do anything against ubered targets ( demos / soldiers / engineers can all knock them or push them back )
Can only really hit the slower targets reliably
And again, Ill repeat this again!
Snipers could be awesome on your team but only if he had aimbot like skillz and only on a map that suited it because lets face it, there arent that many maps with 1 decent Sniping area, let alone 2-3.
All maps are made with all classes in mind ( in theory anyway ) and unfortunately, this pretty much craps all over the Snipers since they are the only long range specialists. All other classes are short/medium range so maps are designed with this in mind.
TF2 came from Quake and is still similar in a lot of ways. If you play Quake or Unreal games, the Rail Gun and Sniper rifle/ Plasma rifle are good but thats only because you can carry all the guns and switch between them. I never played Unreal games that much but I cant imagine trying to deathmatch in Quake with only a Railgun and the crappy Nailgun :/