It seems that Apple painted itself into a corner here.

Apple releases a new iPhone every year and it comes with major-ish OS update. They can't just not ship and wait until it's perfect - there's too much money riding on this upgrade cycle.

At the same time, Apple dropped contract with Google way before iOS 6 was ready.

Since delaying iPhone 5/iOS 6 combo was out of the question and they didn't prepare for the plan B (i.e. shipping Google's maps) their only option was to ship their maps regardless of how shitty they are.