In the case of using Rewire to connect Reason and FL Studio to Cubase or ProTools, yes. In the case of using Jack to connect Rosegarden to Ardour, kinda no. Rosegarden already has Jack support and is also an audio app, like Cubase. Now it's redundant. What the folks coding Ardour should have done was incorporate a sequencer directly into the app. They could have integrated the sequencing code from Rosegarden, thus making Ardour a more well rounded app instead of forcing everyone to use a second app for sequencing.

It would be like rewiring Cubase to ProTools, you can do it, but why?