Hmmm, it does depend though. Its true that drama does not scale as well as other subjects.
My experience? I was desparate to drop Chemisty (final mark 81), and thought I was doing terribly in ext1 English (final mark 36/50), but in the end, those two subjects got counted for my uai, and my 82 in drama did not. It's not so much that drama scales poorly, but that other subjects scale well.
Having said that, I do NOT reccomend dropping drama, at all! It is one of the hardest, funnest, most worthwhile courses on the cirriculum, you will get so much out of it if you try, and you will have fun doing it. If you like drama, stick with it. Who cares about the scaling? It won't make much of a difference in the end, and what you get out of that course will make up the difference anyway.