I'm inclined to the belief that the women-specific bikes are aimed as much at bike shop owners as at female riders. A lot of the things that women typically have to swap out when they buy a bike - handlebars, stem, saddle - come already swapped on a female-specific model. The brake levers are already shimmed for smaller hands. So it tends to save a little time and effort for shop owners, at least in the smaller sizes.
In larger sizes, there isn't that much difference between most women's and unisex bikes, in my experience. My newer road bike was a unisex model. I had the stem swapped for a shorter one, but the handlebars were already shallow drop and narrow enough, and the brake levers felt fine as-is. The saddle didn't work, but then not many saddles do.
Very small riders pretty much have to go with women's models, as unisex models just don't come small enough. I've never seen a 44cm frame in a unisex model.