SUCHITOTO, El Salvador- I must admit that prior to traveling in Central America I would never have thought to use a plastic baggie as a receptacle for a liquid.
When I was on an archaeological excavation at Copan in Honduras a few years ago, the work crew would take Coca-Cola breaks throughout the day. They would pour the Coca-Cola from the bottle into little sandwich baggies, tie up the tops, flip them over, bite off a corner and drink.
The same process goes in El Salvador. Throughout the streets are little stands that sell refresco — a sweet drink made from fruit, water, and sugar. When you order one of these drinks, you are given it in a plastic bag. There is a straw stuck in the top of it for ease of drinking. For the people here, a plastic bag is an acceptable substitute for a cup. I have not yet observed anyone spilling their drinks from one, it is as normal as white rice.
I am also sure that plastic baggies are less expensive than disposable cups.
The use for plastic bags as drinking receptacles is a good idea: they are lightweight, take up little space, and are cheap. If going camping or hiking they could be a good way to transport liquid from a common bottle to your mouth. If having a party that you do not want to clean up after give people bags instead of cups. Or if you are really cheap, you can just drink out of plastic baggies all the time rather than buying cups. When it comes down to it, having liquid in a plastic bag is only slightly more hazardous than in an open top cup.
When traveling, I now keep a few plastic baggies in my rucksack, as they are acceptable alternatives to carrying cups.