I'm sorry, but no. There's no point to adding customs. The vanilla game is already balanced. What's the point of adding customs if they don't do anything, but make the meta more confusing. Characters like Zelda, Palutena, and Ganondorf won't be jumping to a-tier with customs, and top-tiers such as Shiek and Villager also benefit (some times more than bottom tiers). I understand that customs help some characters, but not to the degree that they're actually relevant to the meta. That's also not taking into account that there are still a lot of characters with garbage customs (I know they're being balanced) that pretty much don't benefit at all. *stares intensely at Little Mac* There are also DLC characters that don't get customs at all. What's the point of playing Lucas if I can get all of his benefits in Ness through customs?
Plus how would you even organize customs? Make certain sets of customs like 1233, 3321, 2321, 2222, 3333, etc? That's silly. What if I wanted to use a custom set of 2233, or 3332, or 1313, or 3133? You see the problem? What's the point of giving players the options of customs if you're going to limit there options to a certain set? Especially considering the fact that some players may not like the moves they're are give. What if I'm playing Shulk and I have decisive monado arts in my set, but I wanted hyper monado arts? Do I just get ignored?
Do we only use the best custom moves? Hyper monado is consider to be one of Shulk's best customs, but I've seen a lot of people who prefer decisive monado. So it's obviously a case of opinions. Not only that, but let's say you have a terrible custom and you combine it with a good custom. This allows you to perform infinites, insane combos, and improve ab aspect of a character greatly. This means the custom move that was once considered garbage is now one of the characters best. Yet again, another reason why restricting sets don't work.
But what if we don't restrict sets? Then there's the problem of having to set everything up. You're going to have every player trying to organize what moves they'll want to use during a match. Do you know how time consuming that would be? That's not even taking into account that players usually use more than one character.
We also have to take match-ups into account. Now things just become a huge mess. I have to learn how to play Ryu vs. Marth with 3232, 2323, 1113, 3111, 2333, 3223, 3332, etc? I don't mind having to learn a games meta, but that's just stupid.
That's not even considering the fact that you actually have to unlock these moves. If I pick up a copy of Melee I can start practicing wave dashing right away. If I pick up 4 then I actually have to unlock these moves to get better at the game. And yes you could say the same for characters in some cases, but it's a lot easier to unlock them then it is to unlock customs (that's not even taking into account that unlocking customs is pretty much luck based).