These days, this is the kind of thing I negotiate up front. When they ask me how long something will take, I explain that they can have a prototype quickly, but only if they promise to throw it away as soon as the experiment is done. I explain that they can have me build a movie set or a real house, but that there's nothing in between. [1] And then I leave the choice up to them, explaining that it's really about their business judgement.
Generally people keep their promises on this, although sometimes it takes a little reminding. When they do, the business benefits are substantial. A good product person really benefits from doing quick, cheap experiments. And they also benefit from having a solid platform of high-reliability code for production use. But they can only get the benefits of both if they're careful not to mix the two.
[1] There is actually something between, but they don't want it: http://agilefocus.com/2009/06/22/the-3-kinds-of-code/
Prototypes-become-products is a trope much like _The Mythical Man Month_. We all nod knowingly when it is mentioned, we all know how it will turn out, and then we (well, management dictates that we...) turn right around and do the opposite.