For me, the only issue with RPing is when it interferes with other's gameplay, like when an RPer queues into a dungeon but they are an "Ice Mage" BLM (they only use Blizzard spells) or a Bard who refuses to use songs because their character is a Ranger.

eRP is just creepy though, especially when they are doing it in /say and are doing it in public or, even worse, in someone else's house/appartment. If you're going to eRP, do it in /tells or a linkshell or FC chat.