There are definitely authors out there who cultivate such a distinctive style that their work would never fit anywhere but in their chosen mode/genre. If they're successful doing only this, more power to them, but often, an author who's versatile enough to learn to adapt to other forms or genres is the one who stays busy and employed.
I first learned this lesson when writing for the magazine market. I was striking out left and right because I expected the editors to recognize my distinctive "genius" and mold their magazine around me. (*Snorf!*) Instead, I soon learned, I had to carefully study the magazine I was targeting and learn to emulate its tone and style.
The same held true when, as a normally-single title author, I wanted to expand my audience by delving into the world of category fiction (shorter, "series" books of the type most often associated with Harlequin and the former Silhouette Romance brand.) I had to figure out what type of reading experience the line was selling and try to create a fresh, new story and characters that fell within those more streamlined parameters. In both cases, I met with some success only after learning those lessons.
The willingness to retool your storytelling to fit the available market or seize on an unexpected opportunity not only keeps a writer working but also helps keep the writer from becoming bored doing only one thing. In some cases, it can bring about great career leaps, too, as was the case with two authors I know well, one of whom was asked to ghostwrite a celebrity memoir and the other of whom was invited to submit the first in a series of young adult paranormal romances. Both were challenged to move outside their confort zones. Both harbored doubts about whether or not they could succeed. And both, I'm happy to report, are today New York Times bestsellers.
So if you're ever offered an opportunity to try your hand at something different than the trajectory you'd envisioned for yourself, consider making like an octopus and giving it a try. At its very worst, it will force you to grow as a writer--and that alone may well be worth the risk!