With the right timeline for releases, some people will end up buying 3 copies of the same game. That just makes good business sense.
They can't have a PC version in high-res muddy the sales of the current gen. They can't have a moddable PC version muddy the sales of the next gen consoles.
The GTA series is their legacy, and what they'll be known for for all time. I think they are moving very cautiously after GTA4. Yeah, it seems like there's some online issues, but they are addressing them. By the time a GTA 5 is ready for release on PC, it should have plenty of polish.
We have to see how GTA 5 utilizes the power of the next gen to get a good sense for what a PC version will be like. I'm going to assume they'll recommend 8 thread / 8 core processors on PC to get closer to a 1:1 comparison from next gen consoles. I mean, they can't spend 8 months optimizing it can they? Oh wait