Our fellow poster ShiverMeTimbers, aka Moulton Avery, and his longtime paddling partner Brian Price were paddling cold water in drysuits way back when. They spread their experience of "Rotary Cooling" to stay comfortable in drysuits in warmer weather--that is, by repeatedly rolling their boats and thus chilling down in the cold water to neutralize excess heat. Here was another area wherein a solid roll was helpful indeed, and the repeated rolling kept the skill fresh.
Another benefit of wearing wetsuits and drysuits as the normal response to cold water is purely psychological--by wearing the suit, most paddlers become and remain more conscious, more aware, of the dangers of cold water, and paddle with more prudence and judgment (most people do--Bart Allen Berry and others like him being the exceptions). I know that, when the cold-water season arrives and I switch to the wetsuit or drysuit, my paddling becomes more conservative.
Regarding wearing a wetsuit under a drysuit, I think that one will experience much greater comfort with traditional insulation under the drysuit. As a former recreational SCUBA diver, I'm comfortable in a wetsuit out in open air or water, but the idea of wearing one under a drysuit makes me hot and sweaty or cool and clammy just thinking about it.