I've actually thought about this quite a bit. I can pick up an entirely decent Kokatat paddling drysuit for like $250.
The problems I could see were:
Fit. Paddling drysuits only tend to come in regular (S, M, L, XL) sizes and fit *very* baggy. This would suck underwater.
Waterproofness. I've never managed to stay very dry in a paddling drysuit, for whatever reason. The material is generally just a nylon cloth with a urethane laminate on the inside, vs a scuba drysuit which usually has layers of butyl rubber between layers of nylon. They even make Gore Tex drysuits. While great for paddling, Gore Tex is most certainly not waterproof, especially over time.
Valves aren't that expensive, but then neither are some drysuits. For example, you can pick up a Bare Nex Gen scuba drysuit for less than $700. You can even go to eBay and find a White's Nexus with underwear and boots for $700.
A real scuba drysuit will be waterproof for a very long time. A paddling drysuit probably won't fit well and won't be waterproof. Go to a boating store one day and look at the whitewater drysuits. If you haven't seen one, imagine thin nylon cloth (the kind you'd find as the exterior layer of a jacket you'd get at Wal Mart) lined with a thin painted-on layer of urethane. It just wouldn't work.
