SID mode only engages the arp when you press more than one note, so single notes are treated as a regularly played note.
That doesn’t sound very exciting and I’m sure that functionality can be set up in a DAW, but it’s a lot of fun playing mono synths lines on keys with it.
It’s also useful to fit things like triplets in the old MNM sequencer. Gives you some retrig functionality in a way.
Nice. I can see that being useful. What would be great would be a single note not triggering the arp at all but ringing out normally (depending on your envelopes) so you could get spacious minimal melodies when you want, then more than one note triggering the arp every so often for flourishes 
Okay, I’m super tired today so might not have expressed it well but that’s how it functions, I think haha… Single note = no arp. Multiple notes = arp. Fun when playing regular mono synth stuff then adding the occasional flourish of arpeggiated notes.
That your own software, or someone else’s?
Reminds me of the Matriarch patching: send aftertouch (or whatever you prefer) to arp/seq on/off. Pretty fun 
thanks for the heads up!
I never noticed the secondary functions accessible by Shift before.
I think most of may favourite hardware arps have been mentioned already (A4/DN & Micromonsta for control, Peak’s rhythm control is fun). In software I really like the arp in Alchemy (bundled with Logic), it’s really deep.
My JP8000 has some crazy cool arp capabilities.