This ukemi is used when a weapon is coming at your legs and you can not get out of the way fast enough, so up and over it is. Also used for koshi guruma.
if i am going to jump up, i would prefer to have my feet into the other guy/gail at the same time which would give me a different sort of ukemi. no point of jumping up and wasting an opportunity for attack. i can see the sort of ukemi for kata guruma, but then i'd probably doing something along this line as well http://video.google.com/videoplay?do...76806796015866