This is not true, check your facts! From http://www.mpegla.com/main/programs/AVC/Pages/FAQ.aspx
Q: Are all AVC essential patents included?
A: No assurance is or can be made that the License includes every essential patent. The purpose of the License is to offer a convenient licensing alternative to everyone on the same terms and to include as much essential intellectual property as possible for their convenience. Participation in the License is voluntary on the part of essential patent holders, however.
If someone holds additional patents on H264 and is not part of the licensing pool they can sue individual licensees, they even get a nice list of parties to sue at http://www.mpegla.com/main/programs/AVC/Pages/Licensees.aspx