Religion is useful / Religion is a virus, are both utilitarian arguments because some people care more about what is useful than true AND because some people's standards for what is true is based on judging its utility.

To focus on Christianity (since you brought it up), religion not being useful would be a blow to the "Loving God" claim, as a God who (like a bad parent) gave lousy advice that harmed us wouldn't seem very loving. Similarly, if a utilitarian secular philosophy continuously failed to be useful based on its own utilitarian standards, it's self defeating.

Certainly faiths that don't assert a loving God, and secular philosophies that don't include utilitarian moral and ethical frameworks aren't effected. The usefulness of religion compared to secular philosophies doesn't PROVE one is correct, but if you can DISPROVE one or some of your largest competitors for mindshare, I get why the argument is so stressed.