Why would you ever want to replace a metal valve stem assembly with a rubber one? I'l give you one guess as to which one will fail catastrophically first.
I think that depends. I had this discussion with the owner of my local BMW dealer. Of course he told me that BMW would only recommend their valve stems, which are rubber (actually, a metal "core" bonded to a rubber boot). They have a very low failure rate, especially if you spend the few dollars it cost to replace them at each tire change.
I asked him about the 90 degree metal ones, since they would sure make pressure checks and filling easier, especially on the front. He said he is a fan of those if the wheel is designed for them, and has a brace that stabilizes it like some GoldWings apparently do. He said he would not recommend the 90 degree metal ones on my K75 wheels, but would install them if that's what I wanted. He has seen cases where vibrations at highway speeds have caused the washer to slowly wear or warp and the nut to eventually come loose, even with lock-tite, causing a sudden loss of pressure. Not a fun thing to happen. This is due to the eccentric load the 90 degree bend causes. In a nut-shell, his recommendation was to use what BMW recommends, but that if I want metal valve stems, use the straight ones.
In reality, all valve stems are subject to some amount of vibration, since no tire is balanced perfectly, you pick up some dirt, etc. The 90 degree bend makes it more problematic.
I wanted to add a tire pressure monitor system, so I had them install the straight metal ones. They rely on a nut to keep them in place, instead of just air pressure, so I do intend to check that nut every time I check the tire pressure. Probably just takes a few seconds to grab the stem and try to wiggle it. I think if it don't wiggle, its still tight.
