Problem is history has shown that the mere existence of such information is too much of a temptation to resist. The "extraordinary" circumstances used to justify getting this data has a way of expanding over time until it becomes almost any routine reason being justification enough. The only way to combat this is for the information to not exist.
Yes, in some cases this means a guilty person will go free but we have a long standing belief in western legal culture that it is better for some of the guilty to go free than to punish the innocent for the actions of the guilty. Invading everyone's privacy in the name of catching the small minority that engage in criminal activities is punishing the innocent for the crimes of the guilty.