I confess I haven't read through the entire thread, but as a Steelers' fan I can say that I'd pretty unhappy if McClendon was signed away.
The fact that he didn't displace Hampton doesn't mean the guy can't play. It's a complex defense, and the Steelers' coaching staff places a premium on experience...sometimes to a fault (IMHO), where they leave an aging vet in place a bit too long at times. I liken the Steelers' NT situation to their OLB situation in '07, when Harrison was sitting behind Joey Porter...in both cases the starting vet was aging, but also a longstanding, important cog in the defense that probably stunted the growth of a talented understudy. I'm not saying McClendon will become a perennial Pro Bowler like Harrison, but I do think he has legit starting talent.
The coaching staff is very high on McClendon FWIW. When we drafted Ta'amu, LeBeau himself came out unsolicited to say "don't count out Steve McClendon" to the press corps. In the short time he's played in games, he's shown himself to be very capable and disruptive.
The one thing I will say is that I don't know that he's got a prototypical 3-4 NT build-- at 6'4", 320 he's really ideal as a 5 technique end, just from a physical standpoint. But again he's done well at the nose in limited playing time...I sometimes wonder long-term though if sliding him out to replace Keisel eventually might not be a bad idea.