I think people who are against customs are worried about:
1) "jank"
2) time taken to learn them
3) logistics
4) whatever authorities have said (ie. following).
My responses:
1) Even before we started seriously considering customs, we had a '
dealing with jank' thread here on Smashboards. What is jank? Well, we've always sort of defined jank by what we consider to be a strategy that exists outside our subjective realm of how Smash should be played. When this game first came out, we had many people thinking that projectiles were jank. That went away, mostly. Now people still consider Mii brawler, Sheik, Luigi D-throws, and Ness B-throw to be jank. If something kills somebody earlier than they expected, like Roy's F-smash, suddenly that's jank. It exists with and without customs. Generally, the word jank implies that whatever the word is used for should not be in the game. Players use the word jank when they want to pick and choose what strategies are the good ways to play smash bros.
Honestly, we haven't had results to show that any sort of custom move is 'jank' enough to be banned. The players at evo were able to win against the custom moves. Actually, this is evidence that customs moves are not 'jank.' That being said, just like people who can't figure out how to deal with Link's projectiles, customs will still be called jank by people who are beaten by them, and can't accept it was their own fault.
2) Time taken to learn... well there is
some truth to this. However, it's a very temporary issue. I mean, think about how many moves and stats we know for each character. Aerials (5 moves), smash attacks (3), tilts/jab (4), specials (4), run speeds, air speeds, jump heights, etc... Custom moves are not nearly as hard to learn, because they are already similar to the specials in the game. You won't be seeing most of the custom moves anyways. Then, over time, the issue of knowledge lessens a lot. If we had customs legal from the beginning, we would all already know all of them, or at least the important ones.
Also, you only need your main's customs to participate in customs matches. Actually, you don't need any, but that's sort of besides the point. Tired of seeing people say 'Oh, but I need to unlock all customs!' No... you don't.
3) Logistically, customs work fine. Tournament time is only slightly increased if using the custom moveset project. All you need is a guy who has them on his 3ds to load onto any Wii U that needs it before the tournament. Loading them goes fairly fast if you use the quick button to place and replace all customs. As time goes on, Wii U's will need to have customs loaded less and less, due to Wii U's being reused. As for having a person who has them on his 3ds, there will be more of those over time. If you don't have a person in your local community, somebody can use an action replay power save to save time if he wants. All pretty easy.
4) The followers. This annoys me no matter what the subject is (outside of Smash).
Omni and that European Smash TO both have placed forward unsound arguments. Their opinions were posted in a one-sided manner on the front page of Smashboards. I think a lot of people just read the opening posts but no comments, leaving them with none of the rebuttals or counterpoints.
As for the high level players at EVO who are supposedly against customs (haven't really heard from a lot of them though), they probably would be against customs, for they care about movesets more than characters and want to protect themselves from having to face more viable characters (ie. more competition).
So far, to the majority of competitive players, the customs debate has not been a debate. How can we fix this?