Hmm, I'm not convinced about the "only build what you are going to use" argument. The best example I can think of is pace-makers (or any other form of life-saving equipment). A manufacturer of these devices cannot be a user, because they'd be dead...
I'll bet that a pacemaker embedded software developer who personally knew patients with a pacemaker and hung out with them every day would be a whole lot more motivated.
Just to add to that: I imagine that the makers of the first pacemakers were doctors who were better in tune with the need than the patients. In fact, I'm pretty sure that doctor, not the patient, really counts as the target market. He's the one the diagnoses the problem (and understands the need); he's the one that actually orders the devices; he's the one that actually orders the devices. The patient generally only needs to know that he needs one, and that it's going to be installed.
In the same vein, I'll bet that if actual accountants were to design a piece of accounting software, it would probably rock pretty hard. (Assuming of course, that the accountants in question were also competent designers/developers.)
