Rain tires are the 8th wonder of the world. They are designed for one purpose only, to go fast in the rain.
Rain days at the track are great for all the reasons stated in this thread. However rain tires are built different and use different chemicals and compounds to generate the heat needed for them to heat up in the water. They use a high content of silica (sp) which allows the tire to stick better in the wet conditions.
Generally you don't want to vary either way more than 1 or 2 pounds either way of the manufactures recommended pressure settings. They have already done the testing for you. Put them on, be smooth, and just ride the bike.