Good thing. It turned out quite well for their efforts
That's why the put most of their efforts into Online.
I already explained this in detail on another thread.
If they just left the multiplayer like in GTA IV and RDR the game would have been released in March 2013 like they originally planned because by then, the story mode would have been finished. But I think they wanted to make the online more than that, and that required more time. This probably would have required an even longer delay than the late Sept/early October release, but they didn't want competition from the next gen consoles. So... I think they did somewhat skimp on the single player mode and focused on the Online, which probably took a majority of their time and resources. Even after launch the Online still wasn't complete. They'd rather release it earlier with problems, lots of sales/no competition than later with fewer problems, fewer sales/PS4 XB1 competition.
I ended up explaining it a little... but I just couldn't help it.
These are just my thoughts though.