I realized that my teeth couldn't handle the "feel" of acrylic pretty early on in my pipe smoking. I tried those rubber things, but it is like having a ball gag in my mouth, because it makes the stem so much wider, and makes the stem even more uncomfortable. So, I just try to make sure that I don't buy acrylic stems.I use the “condoms” to protect my TEETH not the stems. All but two of my 32 pipes have acrylic stems, so i don’t worry about disfiguring them.
Some guys use those electrical shrink tubing that they use when joining electrical wires... I think it replaces the need for electrical tape. Anyways, you can find them at the hardware store in the electrical fittings section. You just cut a tube, slip it over the stem, and use a heat gun to shrink it to fit. It is a much better fit in your clench. But, honestly, I don't use them, so I have no first hand knowledge. Like I said, I just make sure my stems are all quality vulcanite.
As for @anotherbob , he's not really a pipesmoker. He's a trained talking monkey in a Tokyo zoo with a stolen cell phone. He was rescued from a house of ill repute where his job there was questionable, but now they rent him out by the hour to tourists who don't like a lot of questions. He just comes on here, because the WIFI limits what sites he can go on.