The writers can't write whatever they want; each piece still has to get approved by editors and becomes the official stance of the magazine once they print it. This is why they got in so much trouble over the rape story.
As far as relevance, Rolling Stone used to be the gold standard for music and pop culture info. They told people what was cool, and people believed them. But they just don't hold the same weight as they used to. Nowadays, I doubt most consumers check to see what Rolling Stone thinks about an album before buying it, because there are much better ways to gather information to make that decision in today's Internet Age. A bad review by Rolling Stone is nowhere near as damning for an artist as it once was.
When Rolling Stone first slammed STP in the early 90s, my reaction was like, "Man, that's harsh. I can't believe Rolling Stone is being so hard on this great band."
When Rolling Stone gives Blaster a lackluster review today, my reaction is, "Who gives a shit."