You've hit on a topic that is almost as religious in nature as what to lube the chain with

I use them semi fanatically because I don't want a bit of grit sitting on the stem thimble to get blown into the valve and possibly cause a slow leak through the valve. Granted the end of the valve is no where near as exposed as the open cup of a schreader stem but I figure why take chances. On the other hand if I lose the cap during a trialside flat repair it's hardly the end of the world and I don't keep a half dozen spares in my repair kit.

But during a thread we had about this exact same topic roughly a year back a lot of folks said that they served NO purpose at all other than protecting the tube while in the box, that anyone using them likely has dork discs as well and that generally their IQ was lower than a snake's belly in a wagon rut for figuring they were worthwhile.

Oddly enough having had a chain eat an entire back wheel on one occasion when it slipped down between the big cog and the spokes I proudly run dork discs on all my bikes. I guess that pretty much answers those folk's opinion of me..... Geez, the sides of this rut sure are high....