If anything it would have to be picking the weapons that are allowed, not the weapons that are banned. There are too many possible unlocks any class could be using even after you choose to not allow a few, and then you end up at the same old rock paper scissors battle (similar to what marmaduke just said) of who has the unlocks that better counter the weapons the other team is using.
Of course, if you choose to allow weapons instead of ban weapons then you run into full time GRU heavies and stuff like that. Weapon/class combos that are too good to pass up so everyone would run them causing the game to be slow and boring to watch/play.
We're in a strange spot here, us 6v6 players, we have a game that we've chiseled down to something that is fast and fun. However, for it to become as popular as we want it to be we would need to water it down quite a bit, and I don't think that's a good idea. Personally, I believe we should continue on with our strict whitelisting because that's what makes 6v6 so fast/fun, and a consistent ruleset across seasons is something you see in every actual sport (and e-sport). Accept that highlander is the more popular format (and be OK with it since it's quite literally TF2 at it's most pure form), that way we don't have to dilute the 6v6 game that we all know and love in a desperate attempt to gain more players.