Another interesting thought someone gave as a possible reason for the Pope wanting to "regularize" the Tridentine Rite (for want of a better term!!): the Pope wants very much to improve relations with the Orthodox Churches, and heal the divide which seperates them. One could say, with some legitimacy, that the Orthodox Churches would have more difficulty seeing the Pope as being sincere when he continues to allow the supression of the older, more orthodox rite within the the Latin Church itself. In other words - heal thyself, and then get back with us!!
Hence, Benedict may have several rather complex reasons for wanting to address the situation with the Tridentine Rite, and getting the SSPX back into the fold may be only one of them.
I agree - there are a host of other aspects to this, some of them actually more important than the SSPX. Personally, I think the SSPX is not going to be happy, in the long run, because they have been used to doing things their own way for quite some time, cultivating their version of the Old Mass (which, the one time I saw them celebrate it, was dead silent, except for the occasional burst of rapid whispering). Probably when the Old Rite is restored or "regularized," there will be standards and guidelines for its celebration, and the SSPX may not like this.