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Jason: I think that if you're in a team, you should be ready to play if needed, otherwise you shouldn't be there.
That said, I aggree that it would be nice to have a feature to allow the captains to if any players are not in the mood to play for a while, or because they'll be on vacations, or busy or whatever. That way, they could contact the player and ask him/her if (s)he would aggree to sign in for a match or not. Of course, if the captain still decides to go ahead and enter that player in the tournament, (s)he could have to face the consequences, like the player forfeiting or timing out in all games, or taking forever to move. But that's something to ponder when deciding who will be in the team.
If you don't want to play for a while you have that option - leave the team for a while. (Of course, if there was a place where you could check the teams you want to leave/enter and just submit it once it would be a lot easier to do).