I guess you've never seen jackets/coats that have two sliders on the zipper. The second slider on the zipper unfastens bottom up.
No, I guess I haven't; the only jacket I've ever seen that would unzip from the bottom up only did so because the zipper was broken (and it wouldn't unlatch when it got to the collar, so that was useless). It doesn't seem like it would be terribly helpful.