If that works for you, great. But generally when I see people "keep it loose" on a prototype that then gets kept when some decision point is hit, they don't act

If that works for you, great. But generally when I see people "keep it loose" on a prototype that then gets kept when some decision point is hit, they don't actually take the time to clean it up to production grade. And personally, although I'm very good at refactoring, I will still go the path of a disposable prototype in the future when I think it's more effective in terms of total effort. Which for me is any time I expect the prototype to yield significant information about both the surface and the substance of what we're doing.