Banning is a crutch for players who refuse to learn.
Ban Kat! Ban Yi! Ban Fizz! Ban Sona! Etc. etc... But they can't ban everyone they want, so Yasuo slips into the game and totally owns everyone on the other team who can't deal with a champion who you can't beat by just going in like a boss and pressing enough keys.
Okay, I can see how it might make things more interesting in tournament play where carefully deciding which champions to ban is an important tactical decision (not sure if Lux would agree when she gets banned), but all the usual suspects getting banned every time in modes like Legend of the Poro King just means players who refuse to learn how to deal with tricky characters never have to. And then when Fiddlesticks goes 30-0 instead of "I really need to learn how to deal with Fiddlesticks" it's "Fiddlesticks needs to be nerfed! Ban him too!"
In fighting games, you can't ban your opponent's character. If a character gives you trouble, you have to learn to deal with it, even if it means (gasp/shock/horror) having to use a different play style from what you're normally comfortable with. Some characters are OP, but players for whom part of the fun of the game is the challenge of learning how to handle those (often predictable) OP characters with their own mains invariably attain an infinitely higher level of play than those who just whine and give up whenever they are confronted with their nemeses.
I know how many people complained about how they couldn't ban characters from Hexakill, but letting them ban Kat (and several others...) every single time in Legend of the Poro King just ensures that they never learn, never benefit from being forced to adapt, and remain trapped in that defeatist mentality.