> but there ain't no way I'm writing unit tests for a throw away coding exercise

I would, if whoever gave the exercise told me they want unit tests.

Not telling that, and then failing people for the lack of tests is fucking stupid imo. At that point it's just rejecting them for not having the right (secret) religion.

usually for a coding challenge i include in the instructions to add unittests. ;)

since I consider the ability to write well designed unittests as crucial.

Just writing a unit test for an existing code would actually tell a lot more about the applicant's skills than a coding interview with vi. Or a task like "here is 100 lines of code, there is a bug there, show me how would you look for it". I wish more companies were doing these kind of coding tasks. Instead, all we get where I come from is "write a simple todo list application in your free time".