I used to be in agreement with you—but I also think Christians, motivated by love of neighbor, have a responsibility to the common good too. I think back to Evangelist William Wilberforce who was the lead abolitionist that brought about the end of the British slave trade.
Not only was he doctrinally and theologically sound, and totally Christ-centered in all he did—but God had called him to a mission with temporal, secular implications as well. I am sure this required Wilberforce to work across party lines, work with unbelievers, etc...for the sake of justice. Just as many believing Christians worked with Deists and unbelievers in crafting the US Constitution.
That being said, you’re right “the common good” is not our ultimate goal—our primary allegiance is to the kingdom of Jesus Christ.
While I love pro-life groups, pro-sanctity of marriage groups, lawyers who fight for school prayer, etc., we Christians need to stick with what the Savior has called us to do. Those other "causes" are mere diversions. I am not saying they are illegitimate causes. But they must not be pursued to the exclusion of the Great Commission.