Over time the skirts of a piston take a real beating. Forged pistons will begin to collapse, cast pieces will fatige and eventually break. It's best to just replace them when the number of hours gets high, can save a lot on the long run.
We just had this discussion at work today. A buddy of mine had an older YZ250 that "Runs great with 4 years on the same top end, I ain't changing it" despite what we all told him. Skirt broke of, piston/rod jammed up, he ended up needing a crank, cylinder, and piston/rings. Proper maintanance would have been cheaper!