A little while ago Matt Curran and Lucy McCarraher had a discussion running on the topic of writing for money. I promised (threatened) to say a few words myself.
I don't think there are right and wrong reasons for writing. Most people probably write from a variety of motives, many probably unacknowledged. Some people write from love of language, others in hopes of fame and fortune, and a great many to show off or get even with people they feel have wronged them. Writing appeals to some because it is a way of being in the public eye in some sense while simultaneously remaining a recluse. Some people write from a sense of fun, others from a sense of outrage.
John Gardner, the sanest of men, once asserted that "no motive is too low for Art." It may be interesting to know why people write, or rather why they claim they write, but it really doesn't matter. All that matters is what ends up on the page.
I certainly have nothing in principle against writing for money. As a motive, it's no worse than the goal (which seems to motivate a great deal of writing) of showing former lovers that they ought to have been nicer to you.
The problem is that if money is your primary goal in writing, you are liable to end up disappointed (go look up the statistics). Be a stockbroker, or a lawyer, or, if you don't like the hours on those, consider becoming a dashing professional jewel thief or a hostage negotiator. All of those offer better prospects--and some of them even have pensions attached.