the official games are piss easy whether you're leveling up a few or many pokemon because so many parts of the campaign's "balancing" ignore at least some amount of the sandbox
the only three possible downsides of it always being on is if you're holding off on evolving party members that evolve by leveling up, which can be annoying
and like
>>56843487 said if you have a mon that for whatever reason needs to be at a low level for something, it can be a hassle
and also traded mons have a disobedience level cap at which point they'll start performing inconsistently but unless you're doing a wonderlocke or some other self imposed challenge that mandates using traded mons, you'll almost never be in a situation where this is a long term problem
but yeah as other anons have said, there's not really any reason that having it off has to be removed, if people want to have it off to make the game harder for themselves then why not let them
also the big problem is that it shares both experience AND EVs, which can fuck up EV training by giving EVs to mons that don't need or want that specific EV which forces you to free up the wasted EV limit with berries
tl;dr: there's situations where having it always on can be inconvenient, removing the option didn't fix any problems, and it's a problem if you're trying to EV train anything for challenge runs or competitive battling because of shared EVs. why it was removed is probably GF thinking having the option to not have it created problems that don't actually exist, but cmon IVs still exist after 2 generations of no hidden power so it's clearly just another case of GF being fucking retarded and breaking what isn't broken while refusing to fix what's actually broken
