This is an important point not just for ideological reasons, but it is one reason why the FUD argument is not symmetric.
Since H264 was implemented in the open with open and large membership to the body, it is more difficult for some company to come out of the woodwork and say, "Hey, we have a patent that H264 rides on, but we didn't know it until now" Note, QCOM tried this with H264 several years ago and got their butt handed to them by the courts (http://www.broadcom.com/press/release.php?id=1037466). People have had 10 years to examine this open specification. I think its reasonable to say that one would have found a violation by now. And almost all the majors are already either licensees or licensors. There's a slim chance that there's a patent floating around in some patent troll portfolio that hasn't been examined, but that's the only real vector I can see of a legitimate surprise H264 patent violation.
Whereas with WebM its been in the public domain less than a year. It could feasibly take a few years for it to be reaonable that anyone who would have done due diligence to have done so. It's completely reasonable that in a year from now Siemans comes out and says, "WebM infringes these three patents we have". A court is not likely to throw it out saying they've been sitting on them.