They're sort of like life jackets for your kayak - inflatable floats that attach to the sides, for use after you've been swamped in bad weather.

The argument for them makes sense, but I've read in a few books that every attempt to stabilize a kayak using external devices (outriggers, floats, etc., ends up having the opposite effect because they just give the waves more to grab onto.

I've seen canoes (Sportspal) with built-in sponsons all along the gunwhales, and those things definitely benefitted from sponsons. I'm not sure that's at all relevant to tie-on sponsons for kayaks, though.

Bear in mind, I'm asking about sponsons strictly as emergency equipment.