They're running a 4-3, but with the exception of Haynesworth, it's still a 2-gap system. Besides, when they were running a true 3-4, they had DE's (Seymour, Warren) who could get to the QB.
Our scheme calls for a SOLB who is solid against the run and in protection. Others call for it to be more of a pass rusher. You don't find both that often. You could argue that Woodley and Matthews are good at both, but then again Pitt and GB have DL that can get to the QB (I think I just overdid it with the abbreviations). And that's only two teams. Hard to find SOLB's that excel in everything.
Bottom line: Barring another team making a ridiculous offer, I think they sign Spencer to another deal.