Roger and Jem.
It seems to me that refusing point blank to write anything which has some kind of commercial parameter is a bit like refusing to go to bed with anyone who isn't your One True Love - and in that definition of Not Your One True Love I'd include a spouse of decades, or someone you really, really like and are attracted to.
Fine, be absolutely ascetic and celibate about it, but you'll miss out on lots of things, including people and places which enrich you, delight you, horrify you, teach you about human beings (never a bad thing for a novelist to learn more of) and so on.
It just isn't as simple as either/or. It's true that maybe one day I'll be seized with a passionate desire to write something completely unsaleable, and then I'll have to decide wether to change it, or write something I can sell. But meanwhile, being paid for what I write - finding a market - means I'm empowered to do more in my writing than if I wasn't being paid.
Emma