Some of the great genre authors--Dan Simmons, Lois McMaster Bujold, Connie Willis, Greg Bear--have been able to maintain their readership through a variety of stories. They've written science fiction, fantasy, historical fiction, space opera, and other sorts of fiction without losing their readership or their brand in the market. Some other genre authors have achieved this as well, in particular romance writers who have written time travel or paranormal romance, or dabbled in urban fantasy. For the majority of writers, though, it seems that choosing a brand and developing it seems to be a wise career move.
Robin Hobb is a wonderful writer, and as an author of epic fantasy she's deservedly successful. Before becoming Robin, though, she was Meghan Lindholm, and she liked to write in a variety of genres. She's as smart as she is talented, and she made the decision to re-brand herself and to stick with it--and her terrific sales figures and devoted readership have proved the wisdom of that choice.
Of course, the smart thing for a prolific author to do is to write different genres under different pseudonyms. The redoubtable Kristine Kathryn Rusch does that, and does it to perfection--romance, a fabulous mystery series, science fiction, fantasy. Her brands are clear, and the reader who buys one of her books knows what to expect.
I don't have a conclusion to draw, unfortunately. It could be that the time for eclecticism has passed, if indeed it ever existed at all. All careers, of course, are works in progress. This is an ongoing discussion between me and my colleagues, and there seems to be no clear answer. I'm not a believer in writing to the markets--they can easily have moved on by the time you get a book into print--nor am I a believer in writing something I wouldn't be proud of, or enjoy reading. It's a lot of fun as an author to write the story that seizes my fancy at the moment. I love reading all sorts of novels--literary, mystery, historical, science fiction, fantasy, young adult--and I have ideas for writing them. Is it indulgence to go off in different directions? Probably it is. And I'll probably keep doing it.