Why don't they just make 720p sets 1280x720? Wouldn't this be ideal since you can have 1:1 pixel mapping with 720p content (HD shows, PS3/360 games, etc)?

Is there any particular reason they choose the bizarre 1366x768? I'm thinking they took the standard 1024x768 PC resolution and changed the horizontal res to 1366 to get 16:9 aspect ratio. That wouldn't make sense though since most people don't use TV's as PC monitors. I mean there are 1920x1200 PC monitors and 1920x1080 TV's.

These bastard resolution 1024x768 and 1366x768 panels are cheap computer-based resolution panels.

Only a very few high quality units are true 1280x720 and those are usually DLP.

Your observation is correct. You cannot 1:1 pixel-map to these displays as there is not native program source that matches these stupid resolutions. Because the internal scaler is always in circuit (it cannot be defeated) there is little point to externally up/downscaling any source. It is best to leave the sources at their native resolution and let the display rescale it one time.