I asked this question some time back too, Pirl, and promotion of air circulation was the only reason I recall. I've never done it, and have never had any issues. My garden is pretty crowded, but I typically just stick Dahlias here and there throughout the garden beds and not all together, so maybe that helps keep pathogens from jumping from one plant to the next. Dahlias just seem to like growing here, perhaps its more important in some areas of the country than others?
I've been removing the lower leaves of my dahlias as they start looking scruffy. Never knew it was something we're "supposed" to do, it just becomes a necessity, haha! Mine are all in pots, grouped together on the patio where I can keep an eye on them.