Wow...you read about bone bruises and the downsides of micro-fracture surgery and you wonder...long term his knee will never be the same again. The question then becomes how much can the Cowboys get out of him going forward. He is definitely on borrowed time (at 30 yrs old) going forward. From what I am reading, the cartilage formed from the surgery is not as strong as the original. Football Outsiders has a rather sobering article on it:
http://www.footballoutsiders.com/ramblings/2007/microfracture-nfl
I say let's move on from what USED to be. I wouldn't touch him. I would go young (draft) and if I must sign a FA, lets get one without injury issues. I just don't like signing FAs over the age of 26-27. By the time your team has a good young core, that same player is on the way out. If this team was a 13-3 team that was going deep into the playoffs, you could sign a difference maker such as Jared Allen. However, you just shouldn't do that when the rest of the team has such glaring holes (offensive guard, offensive tackle, defensive end, 1 tech, 3 tech, FS, SS, OLB, Speed WR, RB, heir apparent to Witten, and a CB that can play both slot and outside when needed., etc..)
So why worry about Allen, Spencer or any other old or injured player. I would be more interested in some of the malcontents that want off their team. But most want $$$$$ and $$$$$$ is not something we have an abundance of at this time.