Just to voice another point of view (not that I really want to participate to this charming conversation) some of us are actually happy that this game is only one ID, because it avoids having double achievements, and having an "unfinished game" if you didn't go through it the first time (which was my case).

I know, it's a lame thing to be happy for, but hey, each of us cares about different things.
I concur they could put a system to choose if you want to install either one of them.
Games like GTA or WoW take so much space than D:OS is still "okay".