The commentary about "thrashing", "towing", etc. is accurate. But another way to put it is operating conditions that create high heat (which the above do) is what causes valve seat recession.

Over the years lots of mythology has arisen over unleaded fuel and valve recession. The primary victim is the exhaust valve (heat thing). What is going on is the valve and the seat momentarily weld together in spots. As the valve opens again it "tears" metal from the seat. It's doing this in very small amounts, but over time it "eats" away at the seat. The only thing that the lead in fuel did was deposit lead oxide on the seat and valve face, i.e. it made them "dirty". As most experienced welders will tell you it's difficult to weld dirty metals. All the stories about "cushioning" and "lubricating" are misunderstanding of the dynamic in place.

The reason that a stainless valve works isn't really related to weight or any other mystical property that some might imagine, it's a matter of dissimilar metals (stainless steel vs cast iron or steel) not wanting to weld together very well either.