If you use Bb major or minor you can use that really, really low Bb at the bottom of the piano which is an especially rich note and adds fullness to the bass.... sounds better than the A next to it anyway. Le Gibet by Ravel is a good example of using this low Bb to it's fullest...
Did Chopin write any pieces in D# minor or Ab minor? I don't see those keys coming up too often either. Ravel's Ondine is in C# major, which is a bit of an unusual key.
Actually, yesterday I was thinking about the keys Beethoven uses in his piano sonatas, and it seems G major comes up almost twice as much as any other key. And he even has one in F# major!