I would add that when you put a person in a position that you have asked them for input, they will feel pressured to give SOME kind of input, whether or not it is actually warranted. You're kind of setting them up to give input in the same way you might set someone up to tell a joke, they feel pressured to fill that space with SOMETHING.
Most of the time when I have built features that came from customers, it ends up being the least used feature of a particular update cycle. Sometimes to the point that we have to reverse or highly modify it later. The reason your customer isn't in the business of making the thing you're making, is because they have no idea how to make that thing.
If you ask a person what they want out of a new Ford car, they are not going to say "Brake pads with a higher durability for longer use," they are going to say "More cupholders wouldn't hurt, and maybe you could offer it in magenta?"
See also "The Bike Shed Problem": http://bikeshed.org/