Composing without a piano, or sequencer to hear immediate feedback, is such an abstract endeavor that I don't do it. There are some guys good enough and quick enough to do it on paper, or Finale. But I've got to hear it, test it out, weigh the options. Mistakes occur and sometimes these are gold. But you will never know if you can't hear them.
Chopin, Ravel, Debussy composed with a piano because their music was idiomatically pianistic. Simple. Stravinsky always composed with a piano, and I've read that it was not always in tune. Hmm, makes you think. If he were alive today, I think he'd be using a sequencer, but who knows?
I know this. Sometimes you need a basic blueprint for a piece and the piano is perfectly suited for that. Even piano music can be morphed into most anything. It's just a tool.