He's played on some really bad Ds in his time. Plus, it's tough on a pass rusher if he's the only real threat to pressure a QB. They can always chip with a back or a TE. If there isn't another guy to take some pressure off, it's almost impossible to get back there.
Lets be honest. McPhee has cost himself alot of money coming back. He would have been at least a 2nd round pick last year, assuming something didn't show up in the combine. Now, he's probably more in the middle rounds.