The patent system is so that almost every non-trivial software can be considered encumbered. Yet the chances of being sued successfully are different. Why will MPEG LA need to push their codec if they could profit from selling patent licenses to WebM users?
I suspect that the real motivation of the H.264 supporters is that the patent encumbered codec will allow content providers and proprietary software vendors to control who is allowed to produce video playback software (at least, they could make it impossible for free software to support the codec), and impose mandatory content protection. That will make the content industry giants closer to their ultimate goal of eliminating all the culture they do not control.