I had Dan Cooke outfit my Prism with a spray skirt several years back.  What Fallguy says about "warmth" is definitely true.  That being said, I haven't really used the skirt all that much since most of my travel occurs during summer months.  I don't need that warmth and I don't really care all that much if it rains.

I think the value of spray skirts for lake-travel-mostly canoe campers can be found primarily in the shoulder seasons.  It's colder then and staying dry & keeping your gear dry is more important. 

I suspect there is something to be said for being more aerodynamic but I don't think I'd buy a skirt with additional paddling speed in mind.  As far as dealing with waves in big water, well, fortunately I haven't had the "pleasure" yet of a real test when the skirt was on.  There's bound to be value there, if you know the odds of big wave splash are significant.

My biggest issue with the skirt is how best to handle it during portaging.  No matter how you deal with it, it consumes more time and can get a little frustrating during a multi-portage trip.  I'm sure there are more effective techniques for managing it than what I use.

My bottom line: I put the spray skirt on when I solo in the months of May and October... and maybe September.

Jimbo   8-)