Interesting question. There are cases with people with bad lid margin disease or bad allergies where I really don't like to use punctum plugs because it just helps to trap that nasty oils and allergens in the tear film and on the eye. Now regarding dust and dirt, I really don't see that as a problem since the excess tears if you have any will just flow over the eyelid margin onto the cheek. Also, Ifor average dry eye cases, I often just plug the lower system so the upper system will still be open. In the end, I'm not really worried about dust and dirt in the eye but rather keeping as much tear volume on the eyes as possible. Think about it another way - plugs often help to increase the amount of tears left on the eye and that's always a good thing if you are in a potentially dusty and dirty environment. If problems do develop, the plugs can just be removed. But make sure to get the type of plugs that are easily visible and easily removed. Avoid intracanalicular plugs or smartplugs. Temporary dissolving plugs are OK as well.
MJK MD
http://en.wikipedia.org/wiki/Meibomian_gland
Usually a warm (not hot) tea bag will increase the viscosity of the oil in the plugged duct and they are easily massaged to unplug them (seek a doctor's personal instruction on that procedure)
Good hydration is a way to avoid meibomian plug ups/cysts. When you are dehydrated they have a higher probability of occuring.
Best Regards