I think you nailed it.
The problem for B. Allen is the lack of position versatility. I don't think any NFL team can afford to have a back up Center that can ONLY play Center. I suppose on paper he can play guard.... he's no doubt something more than a warm body..... but I'm not sure I've ever even see him in that position, so apparently Kromer doesn't think he can (and I don't either).
I just don't see him sticking. But, it's not like the Rams missed on him. 4th rounders have a high probability of NOT making a team and sticking around very long. When one does, you got a bargain.