The problem with other video sources is that the HDTV timings aren't what a monitor will usually get from a PC. This is particularly problematic in PAL-land, as 50Hz are unheard of on PC monitors, although I wouldn't be surprised if the NTSC "not-quite-60-Hz" caused havoc in the monitor circuitry. Of course, the reverse situation is just as bad. TVs expect TV timings on their HDMI inputs, they'll do overscan trimming, etc.
The problem is that what people really want is more pixels, and HDTVs actually have fewer than most monitors.
Monitors as TVs don't work for me because I want more than just me to be able to watch and that means moving the viewing device further back. After moving the viewing device further back it doesn't really matter how much fine-grained detail there is. Whalla! That's a TV! :)
'course I haven't "watched tv" in a couple weeks... but this is beside the point.
I've found TVs to not be very effective second monitors.