I feel like since we're specifically talking about RPs, that makes this discussion a lot simpler. 

The two biggest cons I've heard for using AI art generation are that said art is technically made with stolen images, and the results tend to be on the generic side. 

The former point has been discussed enough, but I guess we can address it here in part. It's stealing in a way. Even if the art is being pulled from various sources to make something new, it's still just a bunch of tiny stolen pieces of art. Having said that, I'm unsure if that's a problem for forum roleplayers. People have been taking artwork and saying "this is my character" since the hobby started. If you don't think taking a random picture and making it your FC is immoral, then there's no issue here. 

As for the latter. I saw someone create Senator Palpatine in the style of Naruto. And it did a nice enough job. His face looked smoother than Palpatine's, and he looked far more like a ninja than a sith lord, but you could see what the AI was trying to do. That's to say it's no stand in for a paid commission or searching your favorite art hub for pictures. But if you aren't too picky, sure, it can get something for you pretty quick. 

While I have some pretty complex feelings on AI art, it's a tool and how/if you should use it is going to be a case by case basis. I would never use it personally, but I also understand not every GM wants to hunt through pinterest for the perfect FC for an NPC, or a picture for a piece of scenery.