Putting the valve halfway on would reduce the flow, but not the
pressure when the washing machine was off. *Now that yhou have a
pressure reducing valve at the house entrance, there is no reason not
to turn the water all the way on when using the washing machine. *But,
after *52 x 5 uses, the valve should not have failed if the water was
totally "clean". *Do you have some grit/sand in the water that would
have caused the smooth surface to become roughened?

I'm not aware of any sand or grit per se, but the water is hard,
meaning that it does have minerals in it. Would that alone cause it
to wear out with so few uses, or is it possible they just don't make
these things like they used to?

When I removed the old valve, I did notice that the elbow for the warm
water connection had more calcium, etc. on it than the cold
connection, but I don't know if that means anything. BTW, the water
heater is 4 years old and is a Bradford White.