[mattwrightkitty]

As much as I'm not a fan of the Star Wars and Marvel cross-overs with Transformers I DO believe they are sustainable! Not only do they represent preferred toy-lines, but they build play-value into character likenesses in ways with which kids can identify.
Combining licenses seems to be something that LEGO picked-up-on decades ago, right after it started coming out with distinctly themed sets (prior to themed sets, LEGO was a building block toy the sets of which were no more themed than 'this is a house' and 'this is a car.') Since then, toy companies nibbled at the idea, but it seems to have been in the past decade that we've seen really ambitious combinations where license is concerned (most notably the SW/TF and Marv/TF crossovers.)
The 80s produced a lot of toys that, though not married by license, resembled one another aesthetically so much that kids couldn't help but combine-play!
Some kinds of 'play' are simply more readily realized in combined-play-value and cross-over toys. It's just natural.
As well, marrying licenses is way to sustain brand-identities across the board, so that none of the big licenses fall by the way-side. Think of the cross-over phenomenon as being like a rope binding cargo on a ship.
The toy industry (Hasbro in particular), in general, is wise to have picked up on this, and has made decent decisions in it's license selections.