Yes, I they are great for paddling but if there is any wind, you can't tell me you wouldn't be out there wishing you had a mast or a kite!
Speaking of kites, you have no doubt seen the kite boards. What if you were to use a smaller version kite with your kayak? Take it with you all packed up early in the -no wind- morning and then use it to pull you home later when the wind comes up! Anyone try that yet? I thought I saw a photo of a couple guys on a real catamaran, no mast, using a kite for power.
I'm pretty sure there are tiny little "fishing rod" masts and square sails for kayaks.
Lots of stuff:
http://www.easyriderkayaks.com/down_wind_kayaks.htmhttp://www.vision.net.au/~jennings/sail/sail.html