It's a fair point, but then, will they know what "prescribe" means in that context? What about other unfamiliar words? Why even use a quote that students might not understand?
I think what bugs me most is that bracketed changes are usually made to make the grammar of the quote fit the grammar of the surrounding sentence (changing first-person pronouns to third-person pronouns, making verb tense consistent, and so on) or to supply missing referents to pronouns. I don't think I've seen brackets used simply to replace words that might be too hard. It seems to be outside the conventions of permissible changes to a quotation.