If your distribution can't install drivers easily and cannot work without those drivers, then your distribution is broken.
Not all graphic cards have sufficient video decoding either.
If I had to guess, the most popular graphics card would be Intel 945. Which has mpeg2-mc, and nothing more.
Everything that a codec does can be done on a CPU.some things just can't be done on a cpu
The only question is how fast and efficient this would be.
There's no question left. It's answered already. It's neither fast, nor efficient, especially at fullrate with proper post processing, frameblending and other full quality playback features enabled.
All CPU driven HD decoders sacrifice quality for speed.
I don't think anyone has seen the kind of CPU/shader hybrid driver that is being proposed for the open source stack yet.