Co-GMs, in my opinion, are really useful.

In large group RPs, such as Bleach or Naruto-based roleplays, there is a lot of ground to cover. You have the amount of people that will eventually join your rp and the amount of character sheets you will have to read. Then there's the fact that some people will get out of hand sometimes, so there's something that a co-gm could help with. There's a lot of little things that Co-gms could help with. Added to that, I think having someone to collaborate on ideas with is another great one, especially if you can't come up with some good plot twists and they do. 

On the flip side of things, if you're in a small group rp, Co-gms can sometimes complicate things. If too many people are in on the grand scheme of things, like GM-wise, then there will be shit that will have to be explained or changed later on. Of course, I think having Co-gms just really depends on the type of rp and what the head-gm feels about the notion of having co-gms.