I make it a policy never to act on a customer request without making sure I understand why they want it. What problem are they trying to solve? What value do they expect to derive from this change or addition (or subtraction)?
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Sometimes you find, if you understand the few underlying problems, you can come up with a single solution that solves the needs of many customers at once. This can shrink your customer request list down to nothing in short order. It is a powerful technique, but getting to the root "why" is sometimes harder than it sounds.
How to ask why? I have a few tips. Read this piece I wrote for the Boston Product Management Association's blog for in-depth discussion. What are your tips for getting to "why?" Write back.