They guys in the platoon are always smoking and drinking, and we don't mean cigarettes and Coca Cola.
Whenever these guys aren't on patrol, they're usually getting drunk and smoking pot. In fact, the division between the guys isn't just between the guys who are loyal to Barnes and the guys who are loyal to Elias, but also between the guys who only drink and the guys who both smoke pot (marijuana) and drink (the "heads," short for "potheads"). The guys who do both are in the Elias camp, the others are in Barnes' camp.
Drugs and alcohol are all over the film for a lot of reasons, and they signify different things. For one thing, drugs and alcohol abuse was rampant among U.S. soldiers during the Vietnam War. Stone's inclusion of this fact in the film is partly for reasons of historical accuracy. Like the soldiers in the actual war, the soldiers in Stone's film smoke and drink to deal with the horrors of the war. Barnes himself mockingly accuses the heads at one point of smoking marijuana to "escape from reality," and he seems to think that is a wimpy thing to do—Barnes himself embraces the reality of the war, so he claims, and yet he delivers this accusation in a drunken stupor. (Hint: that's irony.)
In addition, it's important that the drug use in the film takes place in a subterranean bunker of sorts, nicknamed the Underground. It's essentially a speakeasy, Vietnam style. The guys enter it through a semi-hidden entrance, it's dimly lit (there are lamps and Christmas lights), and there's paraphernalia and alcohol bottles all over the place. The guys do drugs, drink, and dance to popular 60's hits in their off time.
Welcome to the Underworld
The fact that this place is underground, also gives it a somewhat ominous significance. Stone was obsessed with mythology when he conceived Platoon, and in a lot of ways this "underworld" recalls the realms of the dead in any number of ancient mythologies (in the early drafts of the script, Taylor visited the Egyptian underworld after dying in the second act).
The underworld-esque feel to the soldiers' opium den turned dance club is the film's way of casting some judgment on the guys. They are as good as "dead" when they're doing drugs, even if they're motives are understandable. They're hiding, semi-buried, in a place that, for all its deathlike resonances, is preferable to what is happening above ground.