I think they're most helpful when someone is on the fence about a movie or has heard the name but isn't clear on what it's about or what to expect. In that case, a movie review can really help them decide whether that movie is worth seeing or not. They can, also, be helpful when someone has children & wants to know ahead of time what kind of movie they're taking their kids to. It's true that if someone has already decided they want to see a movie (or not), a review probably isn't going to change that. But, the right kind of movie review column can, also, be great for informing people on movies that they maybe haven't heard of yet. In that case, it could help viewers find a movie that they're going to love but, wouldn't have heard of otherwise. So, yes, in the right situation, movie review columns can be quite helpful.
Not really. Everyone has their own preference when it comes to artsy stuff and reviewers tend to generalize their opinions for everybody. Many times they do a sloppy job or are bent on criticizing a work, to appear tough to impress.
I like to get a collective opinion of a movie before I slap down my money to see a film. If the general consensus is the movie is awful, then chances are, it is. With all the crapola Hollywood produces, you gotta have someone in your corner telling you when to steer clear of that giant pile of steaming movie-crap you're about to step in.