I think their primary reason is for sun protection, on a warm day. Most shorty 'dry' tops don't have latex bicep and neck gaskets (neither of mine do), so you are not going to be 100% dry anyways.
When I wear a shorty top and am concerned a bit about warmth, I wear something warm under it, like a shorty .5MM neoprene for that extra bit of warmth.
1 Canoe/Kayak Trailer
|Table of Contents|