It's just easy, intuitive, doesn't require any scaffolding, and gives you just the information you want, exactly as your program sees it, yet with the full power of the language to format it exactly as you want to see it, in the most useful way. A breakpoint will do the same thing sure, but only by stopping your program in its tracks! Dtrace is the only thing I've seen that is as useful as liberally sprinkled printf's.

A unit test will tell you if you know what output a function should give for a given input, that it does. Ermm, brilliant, but what if that function isn't on your code path anyway? And assuming that you already understand the program well enough to create any state the function expects if it's impure. Above poster has drunk too much Kool-aid and is suffering from Golden Hammer Syndrome ;-)