I'm totally convinced that SDs or deloads help progress. The only issue is people use them improperly. Unless you know your body well enough to know when you're too close to the edge and need to SD, then they need to be programmed in sensibly by someone who knows what they are doing. Breaking too frequently or at the wrong times hampers progress.
To elaborate - I've experimented a lot with SD. In my experience, doing them when you're still making progress often results in feeling weaker after the SD. Whereas taking an SD when I've been spinning my wheels or feel close to burn out, I almost always come back to training feeling stronger than before the break.
It's also (I'm sure this is obvious) important to manage your SD properly with regards to diet and activity level.