You're talking about phasing. It's something WoW introduced back in Wrath of The Lich King if I remember right.
Bioware decided against doing this in the development process giving the reason that they didn't want players who were grouping together to get confused or see things differently, ie "I'm standing over by the mail box." "WHAT MAIL BOX!?"
I personally would've liked phasing, but that's the reason that they (Bioware) gave.
I do see the conflict there. But that alternative I mentioned would be decent I think.