I don't see why it wouldn't be an asset for a startup founder to be brimming with confidence in his own product.
Once you appreciate that these things arent much more than educated guesses, you can be a lot more focussed actually determining how people react to your product, as opposed to going off and coding what noone wants.
On the other hand trying to fulfill every customer wish and implement all features users suggest is not every time a good idea. Usually it leads to an over complex product that is not well adapted to "young markets" of "untrained" customers. Is like trying to sell them a dvr enabled TV set when they just learn how to change channels.