It all depends what space you're working with, no? My parents have a Silverstone Sugo SG-06B HTPC for their bedroom. The TV is sitting on top of their chest drawers and the HTPC needed to fit beside it so an AVR form factor case wouldn't have worked for them.
The beauty with HTPC's is they're not a one-size fits all solution. You could mix and match to get something that fits your requirements and not someone else's. While you personally have no use for M-ITX, I'm sure there are plenty of people who think differently and prefer M-ITX because it appeals to their sense of aesthetics.
By the way, I've got an SG-05 portable gaming rig with i7-860 and GTX 460 and that actually feels like it weighs less than the Antec ISK-310-150 with i5-650 using integrated graphics. The SG-05's boxy shape as opposed to the ISK's more rectangular form factor also makes it easier to lug around (fits perfectly inside those $1 shopping bags you can find at groceries).
As for power, 450W should be enough for most single GPU configurations. It's not like you could overclock the CPU heavily anyway. Very few M-ITX boards have sufficient power delivery to support overclocking at all let alone high overclocks with quad-core CPU's.