I mean, I agree, but depending on how customs tend to be utilized and how the tournament is being run, the time it takes could grow extremely quickly as tournament size increases. But it would also depend on things like custom transfer speed (and indeed, whether or not you can just transfer setups or the customs themselves). If you can only transfer setups, then any player who skipped the 3DS version now either has to find someone with a spare custom slot to borrow, or is basically out of luck.
If it's largely just a few common styles, for instance everyone basically plays Default Mario but with Quick Fireballs, that's easy, but if things are more varied, for instance some people also like Scalding FLUDD, some people like only Super Jump and the rest default, and others want their own unique setups, that can become REALLY time-consuming, especially if you may be playing on multiple setups. I'm all for customs on the U version, but for really large tournaments that are, for some reason, limited in the number of setups they have, it can be a big time-consumer to let everyone tweak their own. For that reason alone, I could only see it being actually used for smaller tournaments.
It takes long enough to change controls, get coached, and do finger-warmers as-is.
