However, if you ask your customers "hey, if I build this, will it be valuable to you?" and they say "no, I'm not interested", you should proceed with extreme caution.
I agree. However, the thing is, that there's also evidence that many products have ben successful despite focus groups telling there were not good products. Go figure! :-)
A case in point is Sony. During the 70s, RCA made a focus group to query weather or not consumers wanted tiny portable B&W TVs. Sure enough, the result of the research was that indeed people didn't want this kind of product. By its side, Sony didn't bother making a focus group, they just designed it and starting selling it. They sold thousands in the first year, which motivated them to continue making it, until Sony became the leader -among many of its product lines- of portable B&W TVs.
Another great example of this is the Walkman, fortunately, that history is available from Sony itself