I think there are 2 issues in this thread. Using a PPC as a stand alone playback device and using a PPC as a control device in a client/server situation. I agree MC for a PPC as a stand alone device makes no sense. PPC's have very limited resources and they keep changing with the new generation of phones, etc. Personally, I just use an iPod downloaded with playlists I manage in MC.
As part of a network, a PPC is a very portable and helpful device for home media control. Older PPC's, such as my iPaq are very affordable with large screens. Wi-Fi gives portability throughout a wide area with easy touchscreen control of audio. I use mine outside when BBQ'ing and entertaining as well as throughout the house for audio control - song selection, volume, searching, etc. That's the application I was referring to in my comments about NetRemote.