If you are given one of these less than perfect pints, don't feel bad about sending it back. You don't have to be a beer snob obsessed with craft beer to want a clean glass. Besides the obvious sanitary reasons, dirty glassware can affect the flavor of your beer which does a disservice to the brewers.
Carbonation is an awesome thing for beer. It produces aromatic foam, creates a better mouthful, and prevents a keg from going flat. It also tells you you're drinking a dirty beer. Dirty beer glassware can have soap residue, sanitizer, food residue, and who knows what else inside of it. The carbonation bubbles cling onto that residue as if they are screaming to you "hey, this glass is dirty.
Here's a simple test to determine if your glassware is indeed dirty:
1. SHEETING TEST Dip the glass in water to see if it's evenly coated. Droplets forming in a wet glass are a sign your glass isn't clean.
2. SALT TEST If you sprinkle salt inside the glass and it does not stick to the sides evenly, then it's not a clean surface. Salt will cling onto any greasy film left in the glass. Whether you see it or not, the salt will let you know it's there.
3. LACING TEST When you fill a glass with beer, it should form parallel rings as you drink it. If the lacing is a random pattern or there is no pattern at all, ditch the glass for a new one.
A dirty beer glass is equivalent to a dirty plate in my opinion. If my t-bone came out with the flavor residue from the last t-bone served on it, it's going back to the kitchen, no doubt.

I'm pretty sure wine enthusiasts would not put their prized lips on stemware when clearly some other award-winning sommelier's lips have been.
Links
Hop Topic World, Craft Beer,
Brewer's Association, Cicerone