One common misconception with "laughing gas" is that it replaces local anesthetic. This may be because a patient on Nitrous Oxide feels the local anesthetic injection less, so they may not even recognize that they got a shot. But, whatever the case, most dental work requires local anesthetic. Nitrous Oxide just helps ease the discomfort of receiving the injection.
In my experience, I have found that while Nitrous Oxide is not for everyone, it really helps others.