Ken2,
One of the primary reasons why Amsoil has always stressed formulation oils with a low Noack # is to prevent oil consumption over long drain intervals. Like it or not, I have many customers who run out to the six month filter change without EVER checking their oil level. In some cases this can be as much as 12,000 miles, so if you were using an oil that evaporates readily, you could be several quarts low at that point.
The other point you make about oil thicking is also valid, although not directly. One of the most effective antioxidants is ZDDP, so both Amsoil and Redline use a healthy amount. You want a high level of ZDDP in there to reduce oil thickening and provide wear protection over long drains, but you don't want it burning off and fouling your oxygen sensor and plugging up your Cat. Amsoil has worked very closely with Lubrizol to develop oil additive chemistries that have excellent high temp stability. In fact Amsoil was the first US oil company to use the Noack test on a routine basis - as you may know this test originated in Europe.
Most folks running Mobil 1 change it every 4000-6000 miles, so developing a very low volatility oil isn't as much of a design driver for them. They could certainly do it, but it would be more expensive ....
TooSlick