I'd say in P4's case it is still worth it. Persona 4 is not so much about its plot (which does not mean I think it's bad, don't misunderstand) but about your emotional connection to the characters and its very "lived-in" world. Out of all the Persona games P4's social simulation aspect is by far the strongest and I don't think that part is ruined by the spoilers. I also think that's why P4 is the only Persona game I can play again and again even though I already know every part of it like the back of my hand. It's really good at creating a lived-in environment with likeable characters, making you feel at home in that rural Japanese town... And no spoiler can take that away from you.