Not all heads are made of aluminum. Those heads still made of cast iron have induction hardened valve seats. I had the good fortune to visit the factory where the GM 3800 engine was made and see the hardening operation. I've also been paid good money by Ford to audit their induction hardening practise on valve seats in their factories. The owners manual of my 71 Cutlass makes a point in telling the owner that the engine has hardened valve seats and is okay for running unleaded. Manufacturers wouldn't be adding such an expensive step to their manufacturing process if there wasn't some truth to needing hard seats. So there is something about lead in gas that lubricates - rather than abrades - the valve seats in older engines.
Regardless of the fact that heads now run with hardened valve seats, I agree with you that it's not that big a deal for older cars. Consensus among the Cutlass owners on the Oldsmobile forum is that hardening the seats is only necessary if you run a car hard.
Valve seats are not an emission component and I doubt they are covered under emission warranty.
95 E320 Cabriolet, 131K