GTA4 was a great game but what frustrated a lot of people were the stupidly high review scores - it was not a near perfect game.
GTA4 downloadable content cannot be evaluated with your techniques. In my opinion it was always all about trying to get more hardware sales by having a GTA4 on the 360 that was not only the same game playable on the PS3 but also had - for the hardcore fanbase who knew - exclusive content.
MS will do a huge marketing campaign in time for the new content and will make their money back.
It is also a great way to get the GTA fanbase buying live points and then exploring other downloadable content.