Because it encourages a mindset that finds identity and unity in circumstances, and not in Christ. It implies that there isn’t enough outside of age/life-stage to bind people together, and that nothing is more important than demographic.
The assumption, and I think it is correct, is that we will relate best to people who are like us in the most significant ways. (That explains why there are no church groups for people with green eyes; eye color is not something that matters.) So, if what is primary about a person is that he follows Christ, that will be more than enough to unify him with other Christians. But if something else is primary, we have to create a group around it, so those people have something around which to unify.
The more we’re told that we can only relate to people who are like us in the tertiary ways, the more we believe it, and prefer to be with people closer and closer to our own exact demographic, thinking that it will bring us unity. We risk making the tertiary primary.
In reality, the closer we get to the center of our unity, Christ, the closer we will be to other Christians. DBM blows our differences out of proportion and minimizes Christ. And without Christ at the center, we can’t have Christianity.