Okay here are the reasons why as listed in the Maya livign FAQ..
For one Maya can only output uncompressed movie files which are huge.. Maybe the Mac version of Maya has more flexibility in this case?
What if you render to a compressed movie format onluy to discover it look slike crud? YOu;d have to re-render rather than simple recompress.. you shoudl allways render to a lossless image format.
If you have rendering problems then you might possible lose everythign you rendered rather than just a few frames. When rendering to frames it is easier to rerender sections of it without havign to mess with editting a quicktime movie.
You pretty much cannot use a render farm to render to a single quicktime movie.
So to recap:
smaller files without quality loss
more flexibility and easier to fix mistakes or errors
You will also find that most all production facilities only use frame sequences and you will look like a dope if you don't know how to handle them.
