A better sollution:
once a WZ started do not allow new people to join. If a player leaves then that team will play with 9.
Ofcourse, DO NOT start the WZ untill there are 10 people in each team.
I don't wanna join a loosing game because someone bailed out at 2-0. Of course i will leave too the moment i enter and see the score.
I'm against the deserter debuff too: is not your fault that 5 from your team are beginners and because of them the match is lost, at least in a random match. If you get in as a premade then yes, you should get a deserter debuff of at least 2 hours