I used to write music on piano/electric keyboard. I have been without one lately, and mostly been using trackers, where the input method is rather spreadsheet-like:
This was initially quite a restriction as there is little one can do at the onset of a note; it simply plays with a given volume; all the nuances are gotten to by altering the sounds over time (or picking different samples ::) ). It's also quite rigidly quantized which means that midi input, even when you do have it, doesn't really work.
I purchased one of these recently as a bridge until I can once again afford a more substantial note-inputting device:
The range restriction (okay it has octave up/down buttons) gave rise to the thought that it might be amenable to inputting polyphonic material. I haven't really figured out a way of using it that I'm comfortable with yet, though.
Using a keyboard as an input device is significantly different in feel (and, I would speculate, though I can't quite get straight in my head why) function) to using a piano to compose non-piano music, though.
I find it interesting that Ravel wrote numerous works that are literally 'orchestrated piano music' but yet they don't sound like that to me at all,
And a number of works the other way around, unless I'm mistaken? I find, from what I know of his music, that I agree with you on this point, though.