One assertion counters another. Joel is wrong because "Uncle" Bob says so. Why do TDD zealots sound like religious preachers?
No harm writing unit tests. They serve well for (a certain amount of) regression testing. The whole Test Driven approach to designing software is more dubious (but wildly endorsed by the Agile consultant crowd).
More sober developers seem to disagree.
From Coders at Work
"Seibel: What about the idea of using tests to drive design?
Norvig: I see tests more as a way of correcting errors rather than as a way of design. This extreme approach of saying, “Well, the first thing you do is write a test that says I get the right answer at the end,” and then you run it and see that it fails, and then you say, “What do I need next?”—that doesn’t seem like the right way to design something to me."