Critiques can help anyone if their goal is better communication. However, the level of critique must match the level of artistic / photographic sophistication of the photographer.
The critiques for a 1st year student are going to be totally different than those for an advanced photographer. Likewise, the people giving the critique must be of a level to understand the photograph and communicate to the level of the photographer.
As a photograph is about communication it is good to have a photographer's statement about what they were trying to do with the photo. At that point, you can evaluate your personal feelings as to the success of the artistic statement and whether they were successful.
For example, if someone is trying to make a high-key subject rendering for a certain effect, then it makes no sense to say, "I think it should be printed darker" since that defeats the artist's vision and intent. You must evaluate the artist's intent with how well the intent is communicated.
I personally try to stay away from "drive thru" comments such as, "crop xxx way," "I'd have done it this way," etc. I would rather have a dialog with the photographer and through a discussion of the photo with appropriate questions, let the photographer come to his/her own conclusion as to the success of the photo.
I am also of the opinion that you should be able to defend your work if you really believe in it. This, for many people, is taken as being "ego" rather than a clear vision of what you wanted in a photo and whether YOU think you have been successful and why.
I really like discussions like that. I also agree with several other people here that if you posted photos by well known photographers on most photo critique sites, they would be roundly bashed - especially those by photographers who's work is difficult or subtle.
For example, William Eggleston's work is about ordinary, common things that escape most people as being worthy of being photographed. Therefore, the appreciation of that work is difficult because it at first appears so mundane. Eggelston would be told by most photo critique sites to come back when he learned how to take "good photos." This is a lack of sophistication and appreciation on the viewer's part because they are looking for photos that fall into certain categories that they have learned are acceptable, instead of evaluating the photos for what they are - a personal vision.
That is the crux of the critique bisquit - learning to really LOOK at the photo. To NOT compare it to a certain photographic category; and to not have a predetermined or defined artistic value system.
You must attempt to put aside your personal value system and evaluate a photo only on its merits alone. For a beginning or less experienced photographer this may mean explaining why you have a reaction to something with real reasons.
For an advanced photographer this means stopping trying to "help" through suggested "improvements," but asking whether THEY feel something adds or detracts from a photo and why. It should be about establishing a dialog about the image.