Wow... FOSS fanboys seem to think everything is bad unless it conforms to their way of thinking.
H.264 was a group of companies wanting to find a common standard they could all work towards. They found that many of the important concepts were patented, so they all agreed to license those patents, and make them super cheap for people to use.
Open Source? Sure, nice that it doesn't conflict with anyones copyrights. But, if it implements the same processes as the patented parts of h.264, it is in violation of those patents.
I've looked, and I don't believe it's possible that they have an implementation that doesn't infringe on the patents.
Contribute code to H.264? Sure, anyone can write code to implement it, and share it if they want.
Use it in your software? Sure, for most uses, it's free, and you don't need to worry about patent infringement cases later.
Yes, large-scale use is an issue, but from what I see, that's for people that write encoders or distribution systems, not for the common case of decoders.\
H.264 is a published, standard, specification, like that for C, C++, Posix, etc. You can write software that conforms or not, up to you.
If you write software that infringes on patents, it doesn't matter if you looked at h.264 or not, the patents were already there before that standard existed.
You still need to get a license or conform to license requirements.
p.s. Google doesn't indemnify you for patent infringement if you use WebM... If you get sued, it's up to you to prove WebM doesn't infringe.