Testing is good, but it hurts the ability to churn code quickly. You get stuck with a bad design, because it's too expensive to re-write all the tests.

Now, I think it's good to do a few high-level smoke tests first, to help nut out the interface. (It's much cheaper than having lots of meetings with the boss drawing boxes on a whiteboard, or worse, writing specs). But good test coverage is something that can often wait until fighting bugs becomes a pain point.