Well obviously if you buy a gram of, say, marijuana, its supposed to weigh one gram. That is, one gram of marijuana. Not one gram of marijuana and plastic
However, where I am from (Southern California), it was always known, and accept that with heroin, weight was not what it was sold as. And thats fine. When we bought a "gram" we knew we were buying 0.8-0.9, and when we bought a "ball" (3.5grams), we knew we were getting 3.1 grams. So there was no complaining - we still called them those weights but we knew what they were. Its just what it was and it made things easier to use those names, even if they weren't exact. It could be troublesome/annoying when someone new/naive to the scene buy a "gram," get the usual 0.8-0.9, and bitch about it relentlessly. Eventually they too learned. Or complained to the Mexican national they bought it from and probably got shot (ok, probably not shot, just cut off).
Interesting fact that I learned from the Mexicans I used to buy dope from. A "mexican ounce" is 25 grams (leading to ~3.1 gram eights and 0.8/0.9 grams) because that number divides evenly into a kilo, giving them 40 "pieces" (25 gram chunks, the so-called "mexican ounce"). I thought it was pretty clever when I first heard about it. Works out better than selling 28 gram ounces and coming up with 30-something and a fraction pieces (ounces). So, hooray for mexican ingenuity.
Did I even answer the OP's question? Oh well.