All right, say the liner was poorly installed and only was only run up 1" above the floor properly. During a typical shower slight leakage occurs, but doesn't show all the way through the substructure (say the building materials slowly soak up the water). Unless the pan is filled, it may not be discovered for a long time.

I did my test a while back and went into the crawlspace. There was absolutely no rot or old water stains on the subfloor, but water was dripping out of the subfloor...... I most likely would not have caught any problems had I not filled a pan.

The test may not be required, but...............