The IA is still pretty far away from independent capacity in some key areas. While we might not be kicking down doors in Sadr city, the Iraqis will still our "back office" type help.
Intelligence. I don't have an optic on what Iraqi capabilities are, or ours for that matter. I would venture to guess that on the technical side, we have an edge over Iraqi capabilities.
Airlift/CAS. The IAF is still in its formative stages. Also, for the foreseeable future, development will focus on forces for COIN missions like observation/assualt/theater lift/training vice integrated EW/strike/tanking packages of a modern AF.
Medical. Our combat medical is undoubtably the best in the world, at this point. We definitely owe our Iraqi allies some help in this matter. Plus, selfishly, it does maintain the skills and experienced cadre we've built up over the past few years.
Heavy logistics. Iraqis don't have the infrastructure to import and distribute the bulk beans, bullets and POL that a modern Army needs. Of all the critical defiecencies, this is probably the least critical, i.e. the Iraqis are getting the hang of this. But I'm not a logistician, either.
Force Protection. You've got all these nurses, airplanes, and intel geeks in Iraq. They need people to man the gate. Patrol areas. Rescue trapped forces. They need air support, and air cover. Air cover requires tanking and maintenance and ammo troops.
TCNs to run DFACs. Mail troops. Construction troops. Etc, etc. etc.
So you see, you start with a minimalist idea for a footprint, and suddenly it grows.
Like anything else, its a tradeoff. A small force for force protection duties limits its role as a QRF. Intel geeks need to eat, too, so you need DFACs. All of sudden, a sprawling tent city develops, and your orginal small FP unit is overwhelmed trying to protect a small city, when it planned for a hamlet.