The reason why they choose the number of bans they did (and why I think they wouldn't implement a opt-in) is that with the current system you are able to match two players and afterwards chose one map of them that isn't banned. For that to work every player has to ban less than half the maps. (Or opt-in into more than half the maps, but that would be the same anyway).
They could of course change that implementation, but I think that would just increase waiting times unneccessarily.
They could of course change that implementation, but I think that would just increase waiting times unneccessarily.