If you're adept with all three Pokémon, which I've stated numerous times if you're going to main PT you should be at the very least, then it should be a go to escape option in terms of no longer having any defensive options or if your opponent is giving you trouble with the current one you have out. Just because you're running a character that's out, doesn't necessarily mean that the next Pokémon you switch into will automatically have you at a disadvantage or into a "worse" character.
If you're somehow equally good with all three pokemon, you should be using the one that's best at the matchup you're doing, which even if this game is balanced enough to where every character is viable, there is still going to be advantageous and disadvantageous matchups across the board. By switching you are changing to a character worse for the matchup while being in disadvantage, so not only do you need to escape disadvantage with a worse character, but then have to win neutral enough with this worse character to do the two changes back to the optimal pokemon.
Sure, it's not exactly a foolproof or necessarily safe option 100% of the time, not much in this game really is after all, but it could allow for some potential mix-ups or ability to trade off some damge or to create some distance between you and your opponent.
What distance? Pokemon Change itself doesn't move you at all, and if you get punished and knocked back out you're back in the same position as before, but even worse as you're more damaged and are playing a worse pokemon.
Like others have said, at the end of the day this is all theoretical and we won't truly know until we have the game in front of us. So I definitely wouldn't go off of potential viability given we'll never know until the competitive metagame fully forms and people discover things surrounding the game.
However, going off observations we can make from what's readily available or showcased, it allows for more potential pros than it does cons.
It has more "pros than cons" when you ignore the cons with it and the ramifications of switching to a suboptimal character in disadvantage.
I've noticed your biggest criticism of the character is them being apart of a transformation and having a missing down special and you're harping on the idea that you can't play PT optimally with all three characters. (It's possible, look at how different top level players are able to have multiple mains and secondaries and are able to use them on a competitive field.)
You should observe some actual results then, just how many top players are there that play three or more different characters at a near-equivalent level that consistently win against equivalent competition? In Smash 4 it's pretty much just MKLeo and Tweek, and two of those characters for them are the notoriously overpowered Cloud and Bayonetta, and Tweek's third character, DK, has been fading pretty hard as of late. Additionally observe the poor success rate of players trying different characters from their established main (when such a character does work it usually doesn't work on the same person more than once), and how even with Smash 4's so-called "counterpick meta" where people switched to other characters to get a matchup advantage, they failed a lot more often than not despite the matchup advantage. If being able to train up so many characters regardless of natural aptitude to an equivalent high level was so feasible, then it should have been the norm in Smash 4's competitive play instead of the vast majority of high level players having 90+% of their success driven by a single character or two, and counterpicking should have actually worked a lot more often than it did. And remember this is with the players being able to choose who they play from the entire pool of characters, not being restricted to a preset group of characters.