The stated justification for losing the capability is to align even OS X developers with the iOS way of the world, where GC is apparently thought to be too expensive. Funnily though, Android phones show that GC is entirely feasible on mobile. I guess they should have tried harder.
And the "leaks" possible with garbage collector are usually not your father's memory leaks. The runtime doesn't lose track of the objects, it just can't release them because programming mistake prevents that. It's usually not hard to track down and fix, because you can capture snapshot of the heap and work out which objects are around by large numbers at the end of the day. (I do not think I've ever had to do this, whereas I've crashed production applications because of inadvertently introduced reference cycles. It shows to me which problem is more severe.)