You get 10% damage from the Jinn's debuff, and can gain a 25% bonus to magic crits if you have the Glyph.
If you don't mind not having the above then you can use the Asbee pets in it's place, they actually hit harder than a normal pet of the same level.
As for taking more malice than wanted and waking things, Jinn tends to work in predictable ways. It'll attack if you're attacking or being hit, and when trying to reach a target it'll run around other players (this has pulled agro from an idle mob only once for me).
As for waking up sleeping targets, Jinn will wake them, but only if it's already attacking, if you sleep something before attacking, Jinn doesn't attack until you do.
In Arena Jinn does give away your position while you're invisible though, since it's only transparent and not invisible to your opponents.
Jinn has both benefits and down sides, you should never use it on mobs that have Fear, or on bosses with Roar (or kill Jinn off before they aoe), however it'll always be useful against Roger and Event monsters.