intellectually, I know there is drag, but I can't really see any evidence of drag. I ride a dyno hub all the time now, I'd rather have lights available to me than worry about a minuscule amount of extra work. B&M is coming out with a light that has a usb port on it, but AFAIK, it isn't released yet and I haven't seen any tests of it.

The dynohub with a clutch is available under a number of brands, it's made by Shutter Precision and Velo Orange sold it for a while. It weighs quite a bit more than their other hub, and I don't see the advantage of the clutch.