In the list of effects, it seems immobilized is one of the few that doesn't grant combat advantage. I'm curious about the reasoning behind this. You're restricted to a small area and the Rules Compendium says that the player 'can't reposition itself on the battlegrid' and offers two examples, one, that it has a serious injury to it's legs, and two, that it's stuck in a giant spiderweb.
This sort of thing seems to be exactly the kind of stuff that would give an attacker a pretty significant advantage.