gt,
While I have not worked with Cubase, I have Sonar and had a PA until recently. There is no problem with getting midi into and out of both the PA or the computer. To play it back as a midi file on the board, you just have to make sure you save in the correct format either midi 0 or 1, depending on which board you buy.

If you just want to play it straight back out of the pc, into the board,(rather than a midi file that you load) again, no problem, it works fine, essentially the same as a workstation. The only thing that makes a difference, if you play (record) it all in at once with auto accmp., it's going to record it all in one lump on a track. Some sequencing softwares like, power tracks, have the ability to disect it and put on seperate tracks, while some don't.
Terry