Well, maybe they are stainless steel. The ones that I've worked with sure are lightweight, especially the "can"/nut part of them.
Rubber stems are much more resistant to bending damage than metal stems are, there's no doubt in my mind about that. Rubber valve stems usually leak from just the reason you state...age. Old, cracking, leaking rubber valve stems are usually the sign of two things...new valve stems were not replaced along with the last tires installed or the tires on the vehicle haven't gotten too old to be safe and the stems are following suit. Sure, you can break a rubber valve stem but it'll take a much more severe bend/impact to break it than it will a metal stem. If I bend a rubber stem to a forty-five degree angle it bounces back to it's installed position. If I bend a metal stem to a forty-five degree angle it doesn't seem to bounce back into position.
Most people with the can/metal stems will never have an issue with them. But, if you start having a mystery leak in a tire...check the stem with the soap bubble liquid and give it time to build up that little pile of foamy bubbles.
Here are some photos of the two types of stems.
The rubber stem is all one piece except for the valve core...the sensor screws onto the end of the stem where the star-headed screw is shown. New stems are replaced by unscrewing the screw that holds the sensor on, and attaching a new stem with a new screw.
The small o-ring (shown removed from the metal stem) is the seal for the metal stem. It is slid down onto the stem, the stem is inserted into the rim, and the "can" is screwed down from the outside to create a seal by compression. The sensor on most all of them that I've seen is non-removable from the assembly. To "renew" this type of stem a kit is used that has a new o-ring, valve stem, and can. Care needs to be taken so that the can is torqued to a specific inch-pound so as not to distort the seal. Having said that, if you find your stem leaking around the base or out of the top of the can you may think that simply tightening it up some will stop the leak...it usually doesn't help or it makes it leak worse.
I *much* prefer the rubber snap-in stems, to each his own.
Naturally our Honda Pilot has the metal stems and my 2003 jeep has no TPMS.
