I know this is an old thread:
Running without hardened seats long enough and you could drop a valve and destroy your engine.
You end up with less and less spring pressure and run the risk of pounding the valve through the retainer.
I can provide pictures but don't have the pictures here.
Valve fell, hit the piston, broke off the valves head, destroyed the piston and cracked the cylinder wall.
Also bent the rod and damaged that journal of the crankshaft (bearing welding) though I could clean it up and re-use the crank and I was also able to re-use the camshaft.
Owner thought they had a lifter noise for about 5000 miles before it let go.