Evans is also injured. Not a very good replacement when he's not on the pitch, and hasnt been for quite a while.
Fact is were fighting on 4 fronts with a very shaky backline, where our central defence consists of
Vidic - just back from injury
Ferdinand - Out for most of the season so far, suspectable to injury at the moment
Brown - Fit at moment, but injury prone
O'Shea - Out for a couple of months
Evans - Not played for us since the 8th November due to injury.
Yes, Campbell isnt exactly strengthening the side, but he would strengthen the squad which short term is looking depleted with a very busy couple of months coming up. They may well be better options out there, but he is the first to spring to mind as:
A) he is a free agent who'd probably not demand too much to get back into the top flight and would probably welcome a short term contract
B) He's proven at this level, and whilst he may not be as good as before, wouldn't look miles out of his depth (wheras the next choice on our squad list, Oli Gill, would)
C) Would be available for all competitions, not cup tied in any
So not a crap call IMO, might be right not to make a move, but would be wrong to rule it out without a thought given our current status, and has absolutely no bearing on Evans long term development at the club (might actually help having an experienced player to learn from who isnt prone to brain farts)