They are literally the worst thing a developler can put in a video game, ever.
>Oh, I really want this pokemon, let me weaken it with a move I know won't knock it out-
Critical hit. Pokemon faints.
>I'll keep this pokemon in, I know he can withstand a few more hits, then that will let me set up a-
Critical hit. You now lose the battle.
>Wow, this sure is a fun match, we're so evenly balanced, I wonder how this next attack will-
Critical hit. You now feel like shit.
Forced crits and moves like frost breath are FINE, but holy shit, you did not deserve to OHKO my Aggron with a critical hit fire blast.
You cant even turn them off in comp.