I've read something along these lines as well DAN - except using dark food colouring in dish soap. (And the dish soap washes off in a nice lather when you're done the test.
)
I suppose the most obvious benefit for doing "drip testing" is that you can do it by yourself and view the results when you've stopped, rather than taking a passenger along with a video camera (or an additional car, if you can't see the tufts in the area you're interested in studying).