Seeking inspiration - are there professional watercolor pencil artists and if no - why not?
Look up Gary Greene. He has some beautiful work and also instructional materials available on Youtube, ArtistsNetwork, Amazon and probably elsewhere. He does both colored pencil and watercolor pencil work. Watercolor pencils are a natural extension of straight colored pencil work. CP artists will sometimes melt their cp pasages with various solvents. Watercolor pencil uses a similar technique, except that the solvent is water. As I'm sure you're aware, it's the beauty of watercolor pencil (and also the use of other solvents with straight cp) that some passages will be dissolved and others left untreated, according to the discretion of the artist. 

You can use any medium in a professional art career. The key here is not the medium used but your own creativity, skill, salesmanship and most of all, hard and persistent work. If your favorite medium is wc pencils, then dig in. It's a great medium to use.