Wines by the glass bear some of the biggest markups a bar thinks they can get away with. A bottle that would retail for $10, for example, might sell for $6 by the glass. At about five pours per bottle, that's $30-a 300 percent markup. What's worse, usually the wines are crap. They might even be box wine.
Restaurants, and especially bars, assume that people who order wine by the glass just aren't that picky about what gets put in front of them as long as it's red or white (or, gasp, pink), so by-the-glass selections are usually where they stick the cheapest plonk they can find. Also, once a bottle is opened, the wine is exposed to air and begins to break down. A by-the-glass bottle sits around until it's gone. That means the wine you're drinking could have been sitting open for hours, or even days. You'll get better quality, and a slightly better price, if you find a friend or two to go in on a bottle.
Actually, the bag-in-box wine packaging is responsible for improving the quality of restaurant by-the-glass offerings, for two reasons.
First of all, after opening, the bag collapses as wine is dispensed. No air enters into the package and oxidation of the wine is prevented for many weeks after opening, unlike the opened by-the-glass bottle mentioned in the article.
Second, boxed wines are economical for several reasons. Of course, bulk buying is a better value; but bag-in-box packaging is also far cheaper to produce than a bottle and cork; and the bag-in-box wine is cheaper to ship and warehouse than bottled wine. (And, BTW, it costs less to recycle than glass). Because of the economy, a restaurant is able to offer a higher quality wine for the same price.
Take, for instance, FishEye Pinot Grigio, which gold medaled in the 2007 SF Chronicle Wine Competition (stacked up next to PGs selling for up to $25 per bottle). The FishEye PG in a 750ml bottle sells for around $8. The 3L box sells for $17 to $20. The restaurant can sell a glass of this stuff for the same price as some plonk in a bottle that retails for $4.25.
That's just the beginning. There are excellent imported French wines in 3L boxes that retail for $25 to $40 per box. In fact, DB Bistro on W 44th in NYC pours Dtour Chard which is only available in a 3L bag-in-"tube".