What are a wine's "legs" a measure of?

Hello there! I'm Dr. Vinifera, but you can call me Vinny. Ask me your toughest wine questions, from the fine points of etiquette to the science of winemaking. And don't worry, I'm no wine snob—you can also ask me those "dumb questions" you're too embarrased to ask your wine geek friends! I hope you find my answers educational, empowering and even amusing. And don't forget to check out my most asked questions and my full archives for all my Q&A classics.
Dear Dr. Vinny,
When wine tasting, what are the “legs” a measure of?
—Craig L., Las Cruces, N.M.
Dear Craig,
For those who may not be familiar with the term, “legs” are the translucent streaks that cling to the sides of a glass of wine and drip back down after you swirl it. (I’ve also heard these streaks referred to as “tears” or “curtains.”) They’re caused by the alcohol content of the wine, an interaction of evaporation and alcohol’s lower surface tension.
When I was first learning about wine, I never really understood why everyone pointed out the wine’s legs. Sure, they’re kind of neat to stare at, but the same thing happens in my Bourbon glass. All wine has alcohol, so all wines will leave these legs, and I’ve yet to meet anyone who can analyze them. The legs left by a wine with an alcohol content of 12.5 percent by volume look the same to me as a 14.9. Even if there was a way to read these tea leaves, factors like the air’s humidity (or even just covering the glass with your hand) can affect how the legs are formed. I’m not impressed.
So when I hear someone comment on a wine’s legs, it’s nice that they’re observing them, but it sounds like something a newbie would say. Not that there’s anything wrong with that. We were all newbies at one point, even me.
—Dr. Vinny