Especially in the prototyping phase, I do a lot of iterating and refactoring - and I mean A LOT, because I've found that this is the best way for a good design to fall out. Sometimes like 20-30% of the code get refactored, replacing core data structures and control flow.

If I ever had to rely on unit tests for this, I'd shoot myself, mainly because it would make me 1/2 times slower (because I'd have to reimplement all unit tests every time) and this would really break my flow.

Also, relying on unit tests for refactoring means that having anything short of 100% code coverage (i.e. 70-80% isn't enough) defeats the purpose of the whole thing anyway - and I've yet to see 100% test coverage in a real-world project. I realize this isn't an argument in the context of your reply (it was either test coverage or static types), but still, something to consider.