Piano sketches can be useful for beginners in composition because it keeps them focused on making sure the raw musical material is worth actually expanding into something big like an orchestra. However, once you are skilled enough in simply "composing", and your familiarity with the orchestra grows, you tend to outgrow the sketch and see its very limited utility. I actually sort of dislike the piano as an instrument, because it is really not very expressive and sterile, whereas the orchestra can play from whisper quiet to deafeningly loud and with a huge number of articulations that a piano simply cannot emulate. What can happen if you rely all the time on a piano sketch is that you will orchestrate the music in a very basic manner; no trills, horn rips, portamento, mordents or other ornamentation...because the piano is not really capable of it. Another issue is that in orchestration, tone colour is of paramount importance and basically why you would use an orchestra in the first place. A melody played on the oboe can have an entirely different character than it would played by the horns; the piano captures neither. Where this also manifests itself is in harmony. For example, in a piece I am presently working on, it begins with the horns playing a Gm add9, no5 chord at piano dynamics. This has a real warm, ethereal quality that the piano and sustain pedal couldn't capture. Thus, if I had reached for the piano instead of a horns patch in my DAW, I don't think I would've come up with the harmonic language the piece has. Another thing is that: I compose for film. Two films I did this year I had just a few weeks to compose the score before the premieres in a major city. So there were hundreds of people counting on me to be on time. I just don't have time to sketch things out in a piano and then try to translate it to an orchestra. Especially not in film, where the picture itself influences the structure and orchestration; makes way more sense to just go straight for the instruments you have in mind. You can also get some sample libraries these days which are quite playable or at least set up a couple of patches to play in tandem (like staccato + long) and "sketch" that way, with the instrument sounds right at your finger tips. In notation software, you could play your melody on your keyboard, and then just enter it straight away to the violins. Don't like how it is on the violins after all? Copy/Paste it to the flutes in a second. Piano sketches have their uses. Concert composers might still find use for it and beginners will value it, but modern technology has given us better alternatives, I find. It should also be noted that composers of old often didn't really use a "piano" sketch even if they worked at a piano. Following what @Sojar Voglar said, John Williams' sketches were always 3-stave reductions for winds, strings and brass. So even when he was writing at the piano, he was pretty much orchestrating it at the same time. He was always composing with the orchestra in mind.