In my opinion, Merriman is out of position at 43 OLB.
Coming off an achilles procedure, to a position where he's never played and he'll have to cover and play in space doesn't seem like the best fit. Especially since his greatest/only strength is as a pass rusher.
Also, from Chan:
http://blogs.buffalonews.com/billboard/2012/02/chan-on-merriman-brad.html