The top photo shows a flat spot on the valve stem extension where the extension went through the grommet or stabilizer. It was caused by dirt and grit getting between the extension and the grommet and the constant flexing while driving wore the flat spot in the extension. The remaining thickness in the extension is likely only a few thousands of an inch.

In case anyone wonders why the extension would be worn almost to the point of failure and the grommet not showing any wear the answer is shown by a technique used by folks who sand blast lettering and designs in stone. The grave marker industry makes a rubber stencil with the lettering to sand blast the markings on a headstone. They attach the rubber stencil to the stone and sand blast the letters in the stone. The rubber is unaffected, but the stone is sandblasted to created the lettering, often to a depth of 1/4" or more. The grommet functions the same way. Grit getting between the rubber and extension erodes the extension and leaves the grommet almost untouched.

When the extra weight of an extension is screwed on an inner dual valve stem that lever arm which appears held stationary by the grommet is in fact flexing towards the outside of the rotating wheel by a considerable amount. If I can guess based on the grooves in Roger's extensions and the Balance Masters that flexing could be 1/2" or more.

If you use extensions at the very least check for wear at the grommets often, and understand that even with the grommet the valve stem and extension is flexing and the potential for a fatigue failure of the valve stem at the bend increases.