To underline Slim's point, 24 hours ago I posted a sketch of a design......and there hasn't been a single comment on that, whereas there has been plenty on Sketch-up.
This is probably because that is what members now expect from this particular forum......indeed, I have noticed the odd posting of "Upcoming Projects" in the General Woodworking forum which are really just design discussions. Presumably they were posted in General Woodworking because people know that Design really means CAD in the context of this forum.
As long as people are aware of the prevailing wind when they post, the titles of the forums (fora, fori??) are pretty much irrelevant.
My penny's worth is that if you are designing for yourself then there isn't a great advantage in using a CAD program above a pencil sketch, but if you have to present your ideas to someone else and your drawing skills aren't great then SU or similar can help. Personally, I just can't be so prescriptive in my woodworking.........but can see why other's don't see it the same way. (My dad used to draw every thread on the screws in section on his own working drawings......and that by hand when that sort of thing takes a few minutes!!!!!)
Mike