It's a big hassle because many of the things that cause you to get up from your computer will both distract you and require your hands: spilling coffee on the table and running to get a paper towel to wipe it up with; stopping your baby, in the same room, from touching something they're not supposed to; retrieving the next season-DVD of a TV series from your bookshelf. It would be far better if the computer "failed safe", so to speak.
Auto-lock/"screensaver mode" (wow, remember when computers had screensavers?) sort of does this, but the time the computer is most vulnerable (especially in any semi-public setting) is right after you dash away, not after it's been sitting idle for 15 minutes. When a computer's owner could come back at any minute, the best time for a social engineer to strike is the moment the owner leaves.
The last case especially (watching a marathon of some show with some friends) reveals an interesting bit of etiquette: it's rude to lock your computer in front of friends--it implicitly suggests they're likely to mess with it, and that you don't trust them enough to mess with it in a way that's merely funny, rather than potentially harmful. The great thing about an automatic proximity-based lock would be that, in going to the bathroom or whatever, the computer would always lock--so there'd be no decision to make which could be read into. (This is oddly similar to rhetoric regarding the incentive-structures of birth control pills vs. condoms.)