My fear is that we could change him beyond his creator intentions. In the first stories he was a grim character, coming from a life on street and the killing of his girlfriend. Plus, when he returned brought the news that he had found his son... now he hangs around as a very joyful character...
I wouldn't have problem keeping him around as a supporting character (an organization like Vanguard should have dozens, I think), but only if depowered. With his powers, he is the most powerful of them all, and having him not coming into the mission would make no sense.
The one poster one character rule could seems a stupid thing, but at the least is a way to make our stories easier, without too many characters to understand, remember and render correctly.
I have never understood the B team members, but I can ignore them as they are just a peripheral concept of our Universe. The same could be not applied for the mebers of Vanguard. So, less there are, easier is for everyone to grasp them well. Also, less core characters are around, quicker an interplay not superficial can be created among them.
