For me an arranger goes hand in hand with an on board sequencer. The quick record way of using styles with fill and intros to record a song, as a midi file to me is a very easy way to get a full song recorded.

95% of the times I use the sequencer to create songs from scratch. Sometimes I would create a user style with just the drum tracts, record that in to the sequencer then add the other parts. The time I use commercial midi files is when I want to learn a new song.

I would use a computer software when I want to record vocals. I would transfer my sequence on the keyboard by an audio connection and synch the keyboard to the computer sequencer. So I would have both vocals and separate music tracts on one program.