1) Good players at key positions are what helps get beyond 8-8. Letting a good player at an impact position simply walk wouldn't help Dallas win. Particularly when you take into account that the team would then have to find a replacement for him.
2) Spencer has been good since his first year here. If you are talking about high sack totals only then last year was his best. If you are talking about holding the edge, stopping the run, playing a ton of snaps, QB pressures and QB hurries, then he has been stellar since coming to Dallas. There is a lot that goes into playing besides just sacks. Now that he won't be asked to be back in coverage half the time, it will help even more.
I almost wish he had hit free agency so people could have seen what high demand Spencer would have created. Teams would have been lining up for him.