Since realizing it was an option, I've almost always kept it on. Exceptions being when new content releases, as I want to experience it in its entirety including opening cutscenes, and during a recent event (was it moogle tomestones?) since you wouldn't get event rewards for in progress runs. As others have said, I'm often joining perfectly capable parties. Sometimes I'm told the person I'm replacing disconnected, or that they had to leave suddenly for reasons. Other times its clear I'm replacing a problem player (either someone who caused multiple wipes, or someone with a bad attitude). Very rarely am I joining a bad party, and it seems to be nearly exclusively trials when that does happen.