The reality is that if I choose a champ, no one from my team should have the ability to ban that champ.
The reality is that it shouldn't be allowed if done with malicious intent, but ultimately the option remains open because there is a plethora of legitimate reasons to ban a teammate's intended pick.
Such as:
1 - People don't like to play against or with highly contested picks (like Yasuo, Katarina, etc.) and choose to just perma-ban them instead.
2 - Newer champions (such as Yuumi and Qiyana) are liabilities more often than not, banning a new champion spares your team from first-timing them and potentially ruining the game.
(This goes for when they've just been released, although there's still a brief period afterwards where people don't want to deal with newer champions.)
3: You may be hovering the champion, but that doesn't guarantee you'll get first pick, which just leads back to point 1 as you run the risk of an enemy getting them first.