I completely understand your point, and there is also something to say for it. BUT (there is always a but ^-^)
For example, on 2fort, when their 4 snipers out, just become demo, you walk up to them, fire some pipebombs up, and you finish them off. Do this a couple of times behind each other, and the sniper will get tierd from it, and change class. (Alway's seems to work with me).
The bottom line here is, what i try to say, is that imo there is always a counter tatic availible to annoy people so much, that they will change class to do something about it.
provided you don't get sniped before you get there, or get backstabbed, or get blown up by a rocket, or grenades or something else. Its not only snipers that are on the ramparts other classes drop of them 99% of the time. The point still stands, that while the other team can counter it, the people in your own team CAN'T
Its not about having a team of all snipers thats unbeatable, its about a team of all snipers that makes sure you loose the game because you are effectively being handicapped.
Stop seeing it as something thats overpowered, start seeing it as something thats frustrating for the people who actually want a good game.
I would love to see a max 2-3 sniper and spy limit. And a max of 4 engies. If only because we have a custom amount of people on the server, and as someone pointed out before, this leads to some awkward situations that are either incredibly hard for the opposing team or for the team itself.
Simply said, its not overpowered, it is underpowering your team and you loose because of these idiots that only care about frags and not about actually trying to play together and win the game.
I think we have all seen plenty of the following situations
Dustbowl offense with over 40% of your team existing out of snipers, spies and engies.
2FORT with over 40% of your team existing out of snipers, and engies
etc.