I don't play much spy, but I'll gladly be the 4th sniper if that is what I want to practice at the time. Its casual, team comp doesn't matter nearly as much as the relative level of skill between players. Most stomps happen because one side has a fully functional team, while the other has AFKs, first timers, people being matched from 200+ ping away.

I'll play what I'm in the mood to play, its a game and I'm only on to have fun. It doesn't mean I won't play to win. If I'm performing poorly for my standards I'll switch to something more substantial like a Soldier/Demoman/Scout/Heavy. Medic is a situational pick, I love the class but generally only play with friends because coordinating pubs/casual is like herding cats. And my effectiveness relies on their ability to defend me and capitalize on uber pushes.

(Sorry if this sounds arrogant/asshole-ish) But nearing 3,000 hours played and pretty consistently top scoring, I don't think me picking a class we already had a few of is really an issue. And as for the other three snipers? Well, how else are they going to learn right? Better players = better experience to learn from. Casual =/= Comp, you don't actually lose anything for a loss.