GTA4 autoaims for you anyway, you hardly need the fine-grained control of a mouse to hit people. (Much) better graphics were nice on the PC, and mods added longevity, but bugs and performance issues hampered overall gameplay.
Personally I always saw GTA as a fairly casual game, sit down on the sofa in front of the TV and mess around for a bit, maybe do a couple of missions etc. If it was a more serious game maybe it'd be more relevant on PC, I'm afraid I just don't see it that way.
Either way R* are never going to released on PC at the same time as the console launch, this strategy has obviously worked for them for the previous two titles, why wouldn't they use it again?