i believe hss does not freeze the frame as well as a low powered single pulse of flash. the single pulse is so much shorter than the shutter speed.
i dont know where it is anymore but I saw a shot taken of a dremel bit goign 20k rpm, and the author froze with both types of flash. HSS showed a lot more movement because the duration is longer than a single flash.
i think its more useful if want to use higher shutter speed (for large apertures in daylight for example). but then again ive never had a flash with HSS
edit: found it
http://webs.lanset.com/rcochran/flash/hss.html