I don't see why such a restriction would be necessary. Jinn are many [i]many[/i] things. Some are powerful, some are weak, some are malicious and some benevolent. They're wildly varied, just like humans, with their own minds and lives and everything. The [url=http://en.wikipedia.org/wiki/Jinn]wiki[/url] article is quite good.

Obviously, I wouldn't play a shaytan/ifrit/maarid, or any kind of evil jinn, but a benevolent one, of the type who are infinitely fascinated with humans.

EDIT: Basically, a curious minor jinn who joins the guild on a lark, in order to better understand people.