Guys, the take home message is that it's pretty dependent on your ability.
There's no specific guidelines saying that you "must described the effectiveness of the authors' techniques" and there's also none saying you should boycott it.
If a student wrote a top-tier essay, better than anyone's ever seen, and then they wrote a small passage explaining why the author was effective or not, the examiner isn't going to flunk them or deny the 10 that they deserve.
If a sloppy student who generally doesn't have much control over his writing then goes and and incorrectly critiques the author's attempt to persuade his audience, then you might expect that student to be docked some marks.
Whatever you choose, relax in knowing wholly and fully that the assessors are real people too, and they would be aware of the phenomenon and wouldn't be dicks about marking down someone who analyses the techniques (unless they clearly don't know what they are doing and it's major cause to detract marks).