There is no way you could propose a good argument to put lead back in gasoline. It just cushons valve seats and takes a tiny bit of extra effort to build them tough enough to do without. That's it. The alternative is saving 2 bucks on cylinder heads and in the process coating the entire PLANET in a layer of poisonous lead. Lead kills lots of things, there's a reason they banned it.
Advocating the mass usage of lead is like arguing to re-institute the usage of DDT. Remember that stuff, it worked great on bugs, unfortunately it worked a little too well and killed everything else in the process.
If we burned lead gas for the past 20 years at our current world wide oil consumption rate (80 million barrels a day) there wouldn't be a place in the whole country where you could drink well water.