I have no idea how many people are using, but then I have no idea whether many have figured out how to use it in their workflow. I can only talk about what it does, how I've been able to use it, and discussions I've had with others who are using it.

What it does?
The built-in [PLAY/REC] feature can be used to playback and record MIDI and audio data. You can, for example, record data generated by performing on the Montage. You can use it to playback data from any source, as long as it is a compatible form of .mid or .wav/aiff file.

Often, what you can do with external input to the Motion Control Synth Engine, can only be documented as it occurs (audio record). The PLAY/REC feature can be used to capture both the A/D input along with the results of its interaction within the synthesizer as a realtime .wav - the recorder references tempo. This is important because you can then move this .wav to your DAW. This allows you to then add Tracks to it. Audio recording is stereo 44.1kHz .wav to a USB stick.

The same kind of thing can be done with MIDI recording - MIDI is stored internally in Montage (Song folder). Again, with MIDI you are capturing a real time interaction with the Synth engine. Performances with multiple Arpeggiators and real time interaction with the Motion engine, that generates multiple streams of data on multiple MIDI Channel, can be easily captured in the Montage as .mid file (every MIDI nuance). The time it takes to setup most DAWs to record multiple streams of MIDI data can be significant (or easy) - your mileage may vary by what DAW and your knowledge of MIDI... but as quick as pressing Record -you can grab an idea with the built-in Recorder.

The other day, I was going through one of the tutorials on FM-X (making sure no typos or dead ends occur in the step-by-step) in messing around experimenting with the multiple arpeggiators... I wound up with a particular groove where the sound was doing something I found worthy of keeping... and perhaps coming back to later. I quickly created a Scene snapshot, then started recording myself for about a couple minutes playing around with it via AssignKnobs, etc. Now I have it as a .mid, next week I plan to put some time in and build something from that little idea.

You literally can drag n drop that .mid file, via Montage Connect, directly into your DAW. Again, you can now add additional tracks from there... clock timing is ensured.

Once you get your Montage, as you play through the Factory programs, there will come a moment when you find yourself interacting with a sound (for minutes, maybe hours) being able to just instantly capture what your doing as MIDI or as an Audio file (to a USB stick) is going to used by you... if you remember it's there

It's there, and because tempo and CLOCK can be used - this means you can build on what you capture in the Recorder.

It's like asking why you would walk when you could always just run everywhere.

Maybe you have lots of energy - run for a living - and really do get something out of running everywhere.

Maybe you never want to run anywhere (poor health, shape, or just really laid back).

Maybe you do some mix of both.

Obviously, Cubase and the recorder are different in terms what pieces you need in the same room. And with less options - the onboard recorder is easier to use. But it lacks complexity that is necessary for certain operations - so it's not a complete replacement. But you were asking about the other way - Cubase is not necessarily a quick operation. You have to turn on a computer, load a program, choose a template or build your tracks, etc. Hook up the wire (USB). With the recorder - you just press a button on Montage to jump to the recorder screen - then 1-4 other buttons or gestures to start playing or recording something. So the recorder/player is instant gratification assuming you're perfect in your playing during recording - a better scratchpad for a quick thought than Cubase. But this may depend on your Cubase skills and if the board is always on - cubase is always loaded on the computer - and you have all the templates setup to reduce your "click count" in cubase so there's hardly any difference in recording audio/MIDI in cubase vs. the recorder. --- there will still be situations when you have the keyboard (possibly) and no computer.

So it's not something someone else can answer. Depends.

Great answers, thanks for expanding given I haven't even played with it yet..... this scenario sounds sweet: "I quickly created a Scene snapshot, then started recording myself for about a couple minutes playing around with it via AssignKnobs, etc. Now I have it as a .mid, next week I plan to put some time in and build something from that little idea". That's perfect, because even with one arp, I find interesting ways to interact that I might never duplicate again....killer to be able to store to a scene, to pick up on the "jam" later. I love the interactions with playing with one arp at a time.... especially the Mega voices. Thx team!