As to your original question, Giok/John, I figure it's got something to do with the coalition the UN would have put together with a unified commander vs. Bush's posse, It looks better to be a commander of two countries' forces instead of one, and it looks great to have Tony Blair say, in effect, "Yes, I trust you with the lives of our citizens." In for a dime, in for a dollar, right? (Or is it "in for a farthing, in for a quid?") One wonders if a majority of British voters trusts Bush that much.

Looking at this from a purely practical viewpoint, there should have been a unified command from the beginning, else one hand doesn't know what the other's doing and there's a waste of effort. It's just a part of the lack of planning that this is being talked about NOW, instead of at the beginning. I wonder how communications and control of the different forces' commands was handled during the war.