The arguments for customs being a relevant meta thus far have been "its more balanced." While this is sort of hogwash, since the custom meta does not need a justification to be a valid format, it does make its way in to the debate on whether or not it should be
the format.
This answer will ultimately come to form after EVO. With that said, I am inclined to say no for a variety of reasons.
#1: Customs on is not definitively more balanced than customs off, for those who would suggest such a reason. In order to state this semi-objectively, we would have needed to have formed tier lists that are reasonably accurate and accepted by general consensus for both metagames in order to have even a vague idea on the relative balance comparitively. We are simply not there yet. Anyone arguing this, or using balance for or against the customs meta is doing this through perceived balance, usually using anecdotal evidence, current trends, or outlier examples. Just because Ganondorf suddenly becomes viable does not mean the custom meta has more viable candidates, nor is the inverse true. In short, balance is relative, and we do not have the precedent for comparisons.
#2: Customs is a pain in the ***, honestly. While the methods for setup outlined by @
Amazing Ampharos help to streamline the process, they are time consuming to unlock at home for those without a 3DS and require a community consensus generated list of predetermined movesets. I myself still do not have all of them unlocked, and I know plenty of players who do not either, making practicing for custom moves suboptimal in many cases. Vanilla is simply more convenient to run, more convenient to play, and more accessible.
#3: Something people do not really consider is how customs in some cases depreciate the depth of top level competitive play. If customs definitively offer a wider cast of viable characters, is this more important than a deep competitive experience? Considering that Melee has survived strong competitively for so long with a viable cast of only 9 characters, I would say no.
When most players talk about whether or not customs increase the relative depth of the game, they attribute that to the amount of options a character has in lieu of their alternative move choices. What needs to be understood here is that this should not be our primary focus of competitive player skill. Additionally, many of these moves invalidate or trivialize the other qualities of the character. If Donkey Kong is able to use Kong Cyclone to conquer the bracket when he otherwise would be unable to do so, does it really matter in the end what else his moveset has to offer? I watched Mii Brawler net a kill against ESAM's Pikachu in the middle of Smashville at 29% from a grab. A play like this in vanilla normally is the result of extreme player error or a brilliant outplay. This unfortunately is just another day at the office in the customs meta.
#4: There are future concerns for customs, such as whether or not they will receive balance changes, and whether or not future DLC characters will receive custom moves, that have yet to be addressed. Assuming neither of these come to pass, it makes it harder to justify having customs as the standard.