You're taking that shit too literally. Promethazine is just short for promethazine with codeine. You might say why do they drink that weak shit? It's because they're casual users, not pill heads or H junkies. It won't do shit for someone used to H but if you don't have a tolerance and just want to chill and smoke some weed it's pretty good. Also it's become a
Veblen good, much like some wines cost more than cheap vodka or fortified wines that are stronger. Also a lot of them rap about Tussionex which is pretty good.
Eminem is just using word play, metaphors and analogies. Think about it, he's rapped about killing as a lot of rapper do, do you think he's really killed? Shit a lot of Rock and Country singers also make songs about killing.
Percs is sometimes slang for any IR oxycodone. He's using vics as an example of a drug to use. I do think it's funny with all his money he claims to be addicted to vicodin, ambien and valium. Same with Lil Wayne about codeine syrup. I think they probably used stronger shit and just said they used the soft shit because using heroin or crack is kind of frowned upon in the Hip-Hop world. A lot do, more than willing to admit. Some are very open about it. But who knows, Howard Hughes was addicted to codeine and valium even though he could get anything.