Hey man, under the overhang is my shady area. Sometimes I want some sun, and sometimes I want some shade.
Anyway, yeah. There are a bunch of problems here. ...But I really wanna see it. Man, it would be so cool.
A builder NPC would be very hard to do, I agree. Automated repair would be totally sweet, though. An enemy group would have to conquer a place by attacking it skillfully, rather than wearing the place down when the owner's not around to fix it, and an enemy can't level your sweet house just by attrition in case you have to leave for a trip or something.
On the other hand, building a having a place be safe without your supervision seems kinda... well, lame.
Also, wasn't there like an update-caused bug that made every one spawn like 1 Z level under where they were before? Even if there wasn't, the possibility of something like that making the build NPC building an extra layer on everything is kinda...
It would be plausible to not be able to share claimed areas, I think. That seems simplest, and I don't think it would require a more interface than the "items only" interface Notch seems to go for.
How should the guard respawning work? Even with the NPC block.
Like, say one dies in a fight. If I had a stack in the rehire slot, would a clone pop out ten seconds later? That would be kinda lame. Maybe it could check once every [so and so time unit]. In-game day's what I was thinking at first, but that seems like it'd be easily beaten by attrition - kill a couple each time, and in probably less than a day you've beat 'em all and are thus undeterred.