You're right that MPEG-LA does not indemnify licensees of H.264, no. Licensing doesn't eliminate the risk of adoption. It substantially reduces that risk. (Largely because patent licensing also function as a protection racket.) MPEG-LA will defend their patents, though.

See, it's not gonna be the content producers (like Google) that the patent trolls go after for WebM; it's gonna be the makers of the WebM-decoding graphics chips. I guess it remains to be seen where Google will be in that fight.