Yeah I can see why snipers would not really have a niche in MvM. They do not really dish out as much damage or support as the other classes. Although, from what I have seen in videos, those Robo Snipers sure can be annoying.

General team comp seems to be:
Engineer -Support
Medic -Healing
At least 2 from: Demo, Soldier, Heavy -Pure damage/DPS
Pyro/Scout/Maybe spy - money collection

I've yet to see a really solid use for snipers :/

Please join http://psteamcommunity.yuanyoumao.com/groups/ducksoupgamingmvm
as per: http://www.ducksoupgaming.com/threads/mvm-team-organization-creation-kawaii-♥♥♥♥.3764/page-2#post-62855

How's the regular rocket launcher in MvM?

Engie and medic are the 2 most valuable from what i've seen. The rest is a bit up in the air. But heavy with pushback/killing projectiles and soldier with beggars bazooka both have a lot of benefits

We can always discuss team composition when we sort out who likes playing what.
Personally, I mainly play pyro and soldier. But I do not mind mixing it up by playing a different class if needed be.

What team comp are we going with?
2 Solly, 1 demo, 1 scout, 1 medic, 1 engy
2 Engy, 1 solly, 1 demo, 1 Scout, 1 Medic
1 Solly, Heavy, Demo, Scout, Medic, Engy