I like the idea of being able to attack a rival guild who claimed a mob we were camping, and the thrill of the same thing happening to me would be equally exhilarating. It was one of the things that seemed neat about WoW. For some of the old content, before they made everything a fucking instance, an opposite faction guild could show up and kill you and take the world spawn right off you. Leads to really interesting stuff, like having people dedicated to PvP watching for incoming attacks and being able to handle a dynamic situation. The same sort of threat is what made FFXI fun before they got rid of MPK. I'd rather roll on a server where that option is open, even if nobody ends up PvPing, I'd hate to get to max level on a server and then be stuck not being able to do something.
It's the same reason you don't roll on a PvE server in WoW, it rarely is a big deal leveling but when you want to PvP the option is there, if you choose PvE, it's not.