Why I want it: It's happened to me countless times. I'm building away on a dedicated server, because I often like to build with either none or very little people around. There's no admin, as there usually isn't on dedicated servers. I finally finish when some idiot comes and blocks my entrance with his brick(s).
My idea: You cannot build above someone's bricks without full trust, and you cannot build any closer than 3 to 6 bricks away without build trust.