I think that the way they did it makes some sense...in unison, the keyboard is effectively locked anyways on the DSI synths as playing on the keyboard will not 'steal' the show from the sequence (some other synths do enable voice stealing...e.g. the MicroFreak).
In poly mode, the sequence is often used as a backdrop to work out melodies or pads for accompaniment (I often mix and match some sequencing stuff with a loop in my DAW to 'audition' musical ideas. This de-legitimizes the use of the keyboard (without some arbiter like the record key) to transpose the sequence.
In some ways the OB-6/Prophet-6 platform is awesome, but there are some handicaps using it as a full performance synth.