apple has the advantage of having their own OS they can mutilate to use on their different devices. and a smooth user interface is somewhat apples trademark, amongst others.

also, the iphone is not an open device like the n900, so they can fasten their seatbelts as much as possible to squeeze out the last bit of performance there is.

how is it a question of priorities?
are there any disadvantages of using transitions?
it makes things look better or smoother while hiding obvious loading times (or at least make them look non-existent)

i dont see why anyone would not want to use them.
maybe i'm missing something...