In custom games, people can fight back by not letting you play in their lobbies.
In Pantheon games, however, you are totally screwing over your teamates with impunity.
To me, the solution is the implementation of Arranged Team pantheon games. Once those come about, people will play RT at their own risk.
The way it is now, everybody who is jonesing for competitive play simply has to bite the bullet when their anonymous teamates decide to ditch them.
If you are playing custom, meh. Who really cares if you quit. IMHO, in a game like this custom = casual. Quitters should be tolerated if not expected. I personally strive not to do it... but that's me.
And to be perfectly clear-- I don't think anybody is saying that you have to stick around until your entire citadel crumbles. By all means... let's not waste one another's time.