Problem is, its doubtful there is no AA by choice, more like Rockstar sucks at optimizing and writing graphical engine etc... Just because it's performance blows without AA doesn't mean they didnt add AA option by choice...
It's not so much incompetence as it is writing an engine for the consoles, then porting it to PC without rewriting the whole thing, which they do not deem cost effective. Console games cover up jaggies with blur and such, which is what GTA4 did on the 360. Hitting "p" adds this to the PC release, but it honestly looks worse than the jaggies.