I bought a pack of those when I was starting out. I fit them onto all of the engines I had at the time, and the first time I took them off to fly, I never put them back on.

It probably depends on where you store your planes, but I never really thought I needed them - other then when I just had to buy them, of course.

I would much rather have one for the muffler, to keep the fuel from running out on my carpet. I usually twist up a piece of paper towel and stuff it in. If I run the engine dry after de-fueling, it's really not an issue.