I agree entirely. Whenever I leave out writing unit tests, I end up with a web of messy interdependencies which ends up far too hard to extend, and will always need refactored as soon as I need to add the smallest feature.

Writing unit tests up front forces me to write the code in a way where inputs & outputs can easily be stubbed out, which in turn makes writing messy interdependencies simply too hard!

Basically, if I write with unit tests, it becomes easier to write maintainable code, and harder to write unmaintainable code.