And having 5 bans means you can ban every main of the enemy team, that might be a problem?
Or be able to ban the counter picks to all 5 of your own teams champions.
Also the more bans a team gets the more they are able to completely shut down counter picks, like ban every champion that even remotely counters one of your teams champions so one player on your side can run roughshot through the enemy team with little they can do about it, at least in terms of getting their hands on a good champion to deal with it.
Give out enough bans and you might start to see team compositions that becomes just too strong because all the possible counter strategies get banned, which will lead to perma ban status of at least some of the champions that make up such a team composition.
The more bans you give out the more complicated the game of banning becomes, perhaps its best to keep the number low to avoid complications?