I'm not completely against TDD. However, one thing I've found is bugs are caused by things you didn't think to write a test for (or write your code to handle sans tests). The mundane stuff I see in the vast majority of unit tests are things an experienced programmer almost never screws up. So how do you write a test for a case you couldn't foresee?
My biggest argument in favor of writing tests is to ensure my code doesn't break relative to other people's code. There are many times I've encountered an update to a ruby gem with no major or minor version change that has broken a, usually implied, contract with my code. I say implied, because duck typers, at least what I've seen in the ruby world, tend to be slack about formalizing interfaces. I write tests to protect against that sort of thing.
My biggest argument in favor of writing tests is to ensure my code doesn't break relative to other people's code. There are many times I've encountered an update to a ruby gem with no major or minor version change that has broken a, usually implied, contract with my code. I say implied, because duck typers, at least what I've seen in the ruby world, tend to be slack about formalizing interfaces. I write tests to protect against that sort of thing.