Usability testing has traditionally answered questions about whether a given application or product is easy and satisfying to use. Questions about whether anyone would actually buy the product are usually left to sales and marketing. I tweaked my latest user test and it paid off in a big way. Rather than simply run a test of whether users could order a report online, I dug into the content of the report and had customers tell me if they understood the data, what data were missing, and what else they needed in order to do their jobs. The test ended up being a combined usability test and a needs analysis, something that hadn't been done previously on this product.
I've been dinged before because I didn't run a "textbook" usability test as described by an accepted guru. The thing is, usability testing is so flexible it can be used to answer any number of questions, and the questions in this case were around what customers needed and the actual marketability of the product. The responses were pretty decisive, and the direction of the development of the product is going to change drastically. That's not something that happens often by following textbook usability procedures.