Logically, it doesn't make a whole lot of sense NOT to allow it.
But of course, programming-wise, with the invite/drop-in system and Steam overlay and what-not, that's not exactly the same.
I do hope very much they make it possible, if it isn't already. I do wonder how one would invite friends then on a non-Steam version, seeing how the current invitations work (but while at Larian MP used a system similar to Dragon Commander so I suppose from there...)
