One of the core conflicts for creative artists of all kinds is the tug-of-war between art and commerce. Frankly, an artist needs to make money, and it is preferable to make it from his craft.
A writer who must work a full-time job to support himself will struggle to find the time to work, and often eventually gives it up altogether. On the other hand, being able to write on any project at all can polish valuable skills, and teach one the rules of the publishing industry.
On the other hand, Ive met writers who were clearly working on projects, or toiling away at a career, that was burning out their souls. I remember meeting one such writer. His business card read freelance hack and literary mechanic. Sadly, but not entirely unexpectedly, he was dead of alcoholism within a year.
How to avoid such burnout? Well, in my own career, in addition writing the books I cared about the most, Ive written Batman comic books, a Star Trek novel, and a Star Wars tie-in. In my television career, in addition to writing for Outer Limits and The Twilight Zone, I also wrote four episodes of Baywatch(!)
And never for a moment did I feel that I was selling myself out....