Indeed, I have not been their biggest fan. Yet, there are arguments that support their use in debates. First, they provide a way to express arguments that should not be weighed like a disadvantage and an advantage. Sometimes, a principle is so important that, regardless of the advantage of a particular policy, it should take precedence over other issues. Second, they provide a way to point out serious inequity.
Critiques can identify sexism, racism, homophobia, classism, statism, etc., in arguments.
The result is that debaters learn to think about the assumptions of their arguments and whether those assumptions are justified. Third, critiques give the negative a better chance. Too many affirmatives win debates because it is easy to present a small case that does not link to any disadvantages. A critique allows the negative a chance to attack the affirmative at a different angle and thereby evens the playing field between affirmative and negative teams. Fourth, the use of critiques introduces new theories that would not be discussed otherwise.