Originally Posted by
Carbonfiberboy
The other function of a dust cap is to keep the valve end clean. If you ride in wet grit or mud, your valve will get dirty. Then when you go to pump, you pump a little grit into the valve. This grit can keep the valve from seating properly and then you get a slow leak you can't find or a damaged valve. I've had this happen.
I'm either pumping up a tire after getting a flat in which case it's the replacement tube (with cap) which is clean or I'm pumping up at home before a ride in which case it's easy to clean a muddy valve stem. Could be an issue with mountain bikers who adjust tire pressure along the way depending on surface conditions - but not for road riding. The weight of the cap is not an issue, but it does take an extra step to remove, keep track of it, and replace.
You're absolutely correct about having caps on the tubes in your bag. I've had an uncapped tube cause an abrasion puncture in an adjacent tube. However if you don't replace the cap on the tube after installation, you'll lose it and then what are you going to put on the tube after it punctures, you patch it and then put it back in your carry bag?
I do the patching and putting back in the bike bag at home where I have an assortment of the little valve caps.