Opinion It should not be necessary, any governing body should realize that religion can sway their decisions in a rather negative way. However with religions grip on society it was necessary to put in place laws that curbed, even only slightly, religions effect on political decisions.
Opinion
This is a matter of opinion; however, here are some relevant facts: In the past, a church-controlled government impeded scientific progress. At one time, the only accepted belief was that the earth was flat, because religious leaders interpreted the Bible to teach that. Later, also due to faulty interpretation of the Bible, it was dangerous to state that the earth was not the center of the universe. States whose governments are still controlled by religion include Iran and most other Middle-Eastern countries. Nevertheless, many beliefs of the Christian church (which most people are referring to when they mention "church and state") are not unfair and are similar to those of other religions, as well as secular codes of morality. For instance, lying, killing, and stealing are rarely approved of. The U.S. Constitution forbids any laws regarding the establishment of a religion. Instead, legislators should strive to make laws that enforce a universal code of morality, rather than a religious dogma.
Opinion Religion and government must be separate because our society is multi-faceted with numerous religious denominations and every other degree of belief and non-belief in deities. A government-sponsored religion would choose one and then force the people to obey the doctrines of that religion. Its doctrines would inform the laws, its religious leaders will gain too much power and that type of power corrupts. Religious majorities have historically been extremely unfair to minorities. The separation between church and state must be guarded in order to preserve freedom.
Most of the founders viewed religious interference as damaging to government. Most were deists who believe in the existence of a remote deity who got things started and then backed off, not interfering with the day-to-day existence of humankind. The type of religionists who want to deny church-state separation today seek to impose their man-made religious dogma on the society-at-large. That is exactly what the founders sought to avoid.