In the U.S. a throwable is required on boats over 16' USCG Regulations! I agree that on a catamran they are almost worthless. That is why most cat sailors just tie them on.

You would not be sailing on my catamaran unless you wore your PFD.(unless it was a drifter).

On a larger trimaran 28' sailing aross Lake Michigan at night we wear our PFD's with lights & wistle. NOR requires PFD's to be worn accross the start & finish lines. (Queens Cup, it's as old as the Americas Cup)

I also fell of a 26' monohull as foredeck man when the lifeline attachment point broke. I was wearing a lifejacket but was smart enough to hang onto the spinnaker pole.

Treading water is hard work, make it easy on yourself wear your PFD. I also find that the USCG and Police Marine patrol will leave you alone if your wearing it.