I don't think marriage is necessary for any couple, religious or not.

The relationship between two people does not need sanction from a religious affiliation,defacto relationship has the same rights as a married couple under the law,well here in Australia anyway.
Freedom from religions is a fundamental human right,we shouldn’t be coerced into decisions.

I don't see the need for marriage for an atheist couple. It hasn't any value at all, if anything it would cause problems in the relationship.

Legally it adds a lot of value, and protects one partner in the event of an illness or death of the other partner in the relationship. The ritual and ceremony give emotional credence for those who need it. A permanent commitment is, and should be, a pretty big deal. Acknowledging that you have entered into such a commitment is an appropriate reason to give it its own special importance.