I find that past a certain size, even exploratory code base benefits from having tests. Otherwise, as I'm hacking, I end up breaking existing functionality. Then I spend more time debugging trying to figure out what changed.. what's your experience when it comes to more than a few hundred lines of code?

Indeed, but once you start getting to that point I'd argue you are starting to get beyond a prototype. But you raise a good point, id say if the intention is to throw the code away (which you probably should) then if add as few tests as will allow you to make progress.