This is an excellent question - as a keyboard player, I never understood the appeal of being able to generate a sequence of "notes" without using a keyboard - however, given the possibility that one could use the sequence track to generate a random / repeating modulation pattern, and/or non-uniform pitch values (c.f. "notes"), a light has gone off in my head. (I'm a bit slow, sometimes.)
The DSI Evolvers could do this, and the Poly Evolver keyboard had sixteen rotaries to be able to accomplish this (as does my Waldorf Q, eight, doubled up as envelope / wavetable edit controls). The Pro-2 has a step-value knob for each of sixteen step buttons, which works almost as well.
The only downside to knob-based step entry is that, once you attach a MIDI keyboard controller, you'd need to address the issue of a root pitch note for transposition on-the-fly, if pitch even matters to you at this point....