The best thing about customisable moves is that it just gives characters a bit of extra help in certain matchups or facilitates certain player styles.
Against a combo heavy character as DK? Switch his side B to the much slower version with super armour to give you a tool for getting out of pressure if you use it right.
Against a character that likes to control the stage as Mario? Switch your fireball to the fire orb and you have a projectile that travels much shorter, but stays on screen longer, now you have your own tool to control the stage.
Against a character that will try to outzone you and play keepaway as Marth? Switch your shield breaker to the version that gives it a gap-closer utility (maybe this one won't be great if there's considerable endlag, but it's a nice thought atleast).
Feel confident enough that you want to use your super jump punch as a finisher instead of a recovery? Switch to the explosion jump punch instead of having the one that goes higher.
Want to use your cape as part of combos instead of a gimp tool? Go for the shocking cape.
Never use Kirby's inhale? Switch to ice breath instead.
The custom moves are great, they're like 90% of the reason why I'm still excited for smash. We've had a fast technical smash, we've had a slower paced smash, there aren't many places we can go inbetween there that would make the new game feel unique, but custom moves? They have the potential to really make the game feel different compared to past installments, it's just more options for players to really express their style with a character and help in certain matchups that may have been unfavourable, they shouldn't make the matchup an easy win, just give your character a tool to make it more favourable so you don't necessarily have to change your favourite character just because he's bad against who the other player has picked.
Of course if they're completely broken or take way too long to set up, then they have no place in tournaments, but I'm hopeful that - if they're anything like what we've seen with mario so far - they won't be broken and that sakurai will have some elegant solution for choosing which moves you want to use.