I use mine with a Keystep as well, it is perfect! Being able to quickly switch MIDI channels is huge, as you can switch between different synth sounds on the various tracks on the M:S, somewhat like controlling up to 6 monophonic synths from a single keyboard! Pairs especially well with single cycle waveforms, in my experience.
You can also pass the keyboard’s MIDI signal through the M:S to control other external synths using the MIDI THRU mode. This is great if you want to stack a bunch of sounds on each note, for example you could control multiple external synths and also layer up to 6 sounds from the M:S on each key press. MIDI THRU does pass through polyphony, even though the M:S doesn’t support polyphony.
The big limitation right now for controlling external synths is that you have to choose between using the M:S keys and sequencer (in MIDI OUT mode) or using the Keystep’s keys and sequencer (in MIDI THRU mode), you can’t use both at once. Hopefully in a future firmware update that will be possible. Even with this limitation, there’s a heck of a lot of ground you can cover.
Another thing that seems a bit odd to me, when you press a note on the Keystep the M:S tracks will make sound even if they are muted. So basically the mute only applies to the M:S’s sequencer, it doesn’t mute when there’s incoming MIDI. Not a huge deal as you can just turn down the volume for that particular track if you don’t want to hear it.