No offense, but it is the heat. Inlets are cooled by the fuel air
mixture passing over them while the exhaust is heated by the exhaust
gases. If an engine is run lean for a long period of time you can burn
an inlet valve but it is very unusual and by the time that happens the
exhaust valves are completely wrecked.

The hammering of the valve on the seat is a factor but it is a
significantly greater factor on a hotter valve than it is on a cooler
one.

Automotive machine shops will almost always recommend inserts in all
valves. Of course, they get paid by the insert. They will also
recommend having all valve seats cut when only the exhausts are
burned. They get paid by the seat. Eventually when a head has had the
valve seats cut several times you have to have inserts installed in
for all of the valves.

IMO, the valves are so close together in an A-series head that it's a
bad idea to put inserts in the intake side unless absolutely
necessary.

The best advice is, don't run lean, run a bit rich. Fuel is cheap
compared to machine shop time.

Cheers,

Kelley