This ignores the fact that H.264 often has full hardware acceleration in the CPU or GPU of nearly all modern desktop and mobile platforms, which also covers set top streaming devices.

WebM can often use some portions of this (colorspace conversion, etc.) but until the entire stack is natively supported, there will always be battery life advantages to using H.264.

Not to knock WebM - there are projects out there to implement encode/decode in hardware (http://www.webmproject.org/hardware/ for one example) - they're just lagging about 5 years behind H.264 in this, and getting CPU/GPU vendors to dedicate space on a chip to a less codified standard will be difficult at best.