Drop tuning can make sense for a guitar, if the singer is in the same register as the guitar, and they want some separation between the guitar and the vocal. This opens up a few lower notes for the guitarist. It makes sense. And, some guitarists actually like the extra flexibility/bendability this gives their strings.
Drop D tuning is drop tuning only the E string, and while it opens up some new possibilities within a given position, it's a whole different thing. If you create some riffs that work best with a drop D, and it works for you, that's great. But, otherwise, it makes more sense to me to just drop the tuning for all four strings, and only when there is a real need for for me to play in a lower than standard register. More often, I end up shifting everything up two frets, to the actual key of the song, and it doesn't serve any purpose, save to massage someone else's ego.
The funny thing, for me, is that the only times I've been asked to drop my tuning has been in support of a female vocalist, that can't come anywhere near my sonic territory. I mean, if Koko Taylor asked me, I'd oblige. But, otherwise, it makes absolutely no sense to me.