I rarely find a nonbeliever's criticism of Christians to be valid. This one doesn't impress me either. You can't understand the Beatitudes without the Ten Commandments.
The Beatitudes are all traits of Christians. We're supposed to be meek before God but brave before men. We should hunger and thirst for righteousness. We should be merciful peacemakers who are pure in heart. We should expect to be persecuted.
As a Christian, I think it is important to listen to and thoughtfully address/consider all criticism of Christianity. What has caused the critic to think this way? Has a past experience with a Christian or with a church caused this? What can we as Christians do to address the concerns raised?
If we do not address these questions and throw the questions or critic aside as invalid or a lost cause, what good will that cause?
The Beatitudes are, as Jesus is, the continuation and fullfillment of the law of Moses and the Ten Commandments. Both are important on their own, but together within the context of what they represent and stand more are greater than on their own.