So in scenario where two teams filter target bans and a game bans at least 7 bot lane champions one game. That would mean you're out 9 bot lane meta champions in the series, effectively crippling what the targeted player(s) would be in tune with on the given patch. Not only that, teams could just strip more Marksmen even further until none are left at the expense of one game. That's ludicrous if that were to happen in a competitive game. Enough of that and you wind up having some kind of Master Yi/Sona duo lane in the series. What good is that for the players and teams really?
Same thing would apply to target banning out any other position, especially the jungle, where you could wind up in the realm of Karma, Leona, and Nautilus picks. And who would actually practice that on a competitive global scale. Yes I'm aware there are a small few that have unorthodox picks such as AFs Spirit
Even still, the teams could just ban everything not really of use and pick down the tier line for at least 3 games. All that would do is boost the visible percentage of champs not and never picked nor banned.