I don't like to hide what I drink. I prefer to walk around with the A&W Root Beer can in my hand for all to see. I don't feel like tipping a bartender to placing my root beer in a fishbowl glass with a paper umbrella coming out when I can grab a cold can from the table myself.
There are some people who prefer not to drink, or prefer to drink only a little bit. Nursing one drink the entire night might make it a lot easier for some of them to refuse other drinks. They can always point to it and say they're not finished with that one yet. Not everyone feels they can get away with just saying no.
I believe this issue is more of a problem in European countries. They have lower drinking ages there, and it is more customary to have alcohol with most meals (or at every party). In the US, the population of folks who don't drink is probably rather large compared to Europe, so there is a different amount of social pressure.