I'm currently having a discussion, via correspondence, with my tutor on marketing versus art.
With over a hundred thousand book titles being published every year in the States, it's difficult, if nigh impossible, to get your book noticed. If you are a well known series author, or even a becoming-known series author, readers, according to some amorphous research, will stick with the books you write, but not necessarily anything outside of that series. I'm thinking this is why authors write under numerous nom de plumes in various genres. The author is known in those fields, the publishers know those names and genre and run with it because they are successful.
Are series formula, though? We know publishers go with what's successful, not what might be better written or new or fresh. Does formula writing interfere with a writer's creativity? No, I don't think so. A writer can write to formula and still be creative. Witness the romance genre for example. We all know there's going to be a happy ending (generally speaking), but it is the story of how the hero and heroine get there that's important.
Every genre has a type of formula, it is simply a matter of how we, as authors, apply that formula to our own creativeness.
Individual style and content is at issue though. I've yet to make a decision on whether I would, for example, change the ending of a book simply because the publishers found the nationality of the villian to be unacceptable (as one Australian author was asked to do). And if, perchance, a series of books I'd written were so successful, would I want to continue writing that series until I'm dead and buried? I think I'd go the way of many authors and find a pen name to write in a different genre, but still maintain the series.
I think, when it comes to marketing versus art, the marketing machines have misunderstood what's going on out there in readerland. With the advent of weblogs and pages, authors can spread their words further and further afield, network with other authors and readers, promote their work in their own words for less that the marketing budget. It's an effective way to become well known. Diana Gabaldon started her career from a writers group. And like a pebble tossed into a pool of still water, the ripples of her career have expanded to encompass a very successful writing career. Art has triumphed over marketing. The publishing houses have just made it more difficult to succeed when the push particular books, trying to decide what you should and should not be reading.
I think art will continue to succeed as long as authors are willing to work for their talent and readers are willing to read what's been written. It's basically a grass roots process, and nothing works as well as a well fed grapevine.