Its much nicer being able to mix the sounds in your beat then having on mixed instrumental track.
I say this because there could be a sound in the beat jumping out thats competing with your vocals and theres not much you can do if its a single track other then try to eliminate the frequency some, but then your taking from the whole beat.
I too work with a single mixed file most of the time, but It definately limits your mix.
But you can get good results...