Hard to vote for just one. Here are all the CPUs that are regularly involved in "home theater" in our house:
1.83 core duo (connected to living room HDTV)
dual 1.25GHz G4 (MDD FW800 as main HT workstation)
1GHz G4 (MDD FW800 as file server)
867MHz G4 (12" Powerbook as VNC controller)
450MHz G4 (cube in a bedroom for iTunes)
That's 4 to 1, PPC to Intel. Every Mac except the Powerbook is connected through a gigabit switch.
Of course the Intel does the heavy lifting in terms of HD playback.
For me, it isn't so much that "yesterday's platform" is more popular--it's just a recognition that 1) Apple hasn't offered anything as affordable and expandable as those MDD FW800 PowerMacs since and 2) as long as I have an Intel Mac as an extender there's little reason for me to upgrade anything else to "today's platform."
iTunes, ripping/burning dvds, recording high def, managing and serving up content over gigabit--all just fine on PPC.