Buehrle is still young relative to the league, and let's be honest here, we're all hoping Lester can turn into a pitcher as good as Buehrle has been. It will probably happen, but why not remove the doubt and get the real thing? And Coco is finally showing us the game we thought we were getting. He won't OBP as high as Ellsbury probably will, but he'll hit for much more power. They have a looming redundancy in CF. You trade off redundancy to make up for other gaps, and with Schilling being uncertain, Wake being Jeckyl and Hyde, and Tavarez, there is a gap in the rotation.
Now, I think in order to give up Lester and Ellsbury, they need to hammer out an extension, because that's too much talent for a 3-month rental. But if they can work one out in a 72-hour window, that's a good move, IMO.