It doesn't *have* to be a dedicated PC or even one PC - the front (display) / back (record/database) ends can be split.
Then you have to buy two more computers: one to use as the back end and one to use in the living room as the front end. Or which pre-configured MythTV back end device for recording OTA video and which pre-configured MythTV front end device for the living room should a non-technical user buy?
Download and install MythBuntu
Onto what computer? Can someone install it on a household's only computer without A. losing ready access to the PC applications on which one depends or B. harming the recording in progress when the computer restarts for security updates? Non-technical users expect home entertainment equipment to be as reliable as a DVD player, cable box, or unmodded video game console. No, "get your ass off Slashdot if you're not a technical user" is not a constructive answer because if MythTV is not for non-technical users, non-technical users will continue to fill the coffers of the pay-television establishment.
MythTV even supports USB and Firewire devices
All makes and models, or is it like the "winmodem" era where Ubuntu has no driver for a lot of the devices out there?
and can even use your cable decoder, if it supports USB/Firewire
Even if the cable box encrypts the FireWire output with DTCP?
(which, I believe HD units are required to by law - in the US anyway).
"Sure, but HD units require HD service, which will be another $18.00 per month."