More to the point, Phillips' argument fails because it separates faithfulness in marriage from the broader question of the virtue of faithfulness in general. Those of us who are Christians would say that faithfulness is a virtue because it reflects the nature of God. God was faithful to His people Israel despite their repeated lack of faithfulness to Him. God is even so faithful as to die on a cross for sins that He did not commit so that we could have eternal life. But one does not have to make a theological statement in order to recognize faithfulness as a virtue.
Human society is a community of trust.
For civilization to survive we have to be able to rely upon one another to some degree. No community can exist without some measure of trust. We can only pass so many laws, hire so many police, have so many security systems, carry so many weapons. Trust may never be perfect, but it cannot be entirely lacking. Without some measure of trust in those around me I would be forced to look after my own interests without any thought whatsoever to the welfare of others.