You've made too broad a statement. It depends on the woman and the frame. For my very petite wife, she simply can't fit on a men's frame of any size. First, no one makes a frame small enough for her in a "men's" frame. Kids frames with 24" wheels, maybe, but not with a more "normal" wheel size. She currently has a Terry Symmetry...well, 3 actually. Two of them are the same frame with 650C wheels and the other is the classic Symmetry with the small wheel in the front and 700C wheel in the back. She has been through all kinds of other bikes over the course of 30 years trying to find one that is the "right" size. She also has a Trek WSD mountain bike that fits better than any mountain bike she's tried before.

The fit for a small woman is very difficult and the women's design bikes make a very large difference for her. Taller women may not see as much of a difference but it could still be enough to make the bike more comfortable.