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Not necessarily. What if you have a discussion with your religious friend, and convince him God isn't real? You have just saved him/her a great deal of time and money.
For christians, christianity places emphasis on the teachings of Jesus (e.g. treat others the way you want to be treated, etc), not on the existence of God or the beginning of the universe.
People can learn much from how each person responds and supports their response.
There are many "exclusive truth" claiming organizations that argue they are the only way to communicate with God. (Mormonism, Watchtower/Jehovah's Witness', cults in general) If God were proven to not exist these organizations would cease to exist saving millions of people from being manipulated.
It's not possible to ever conclusively "prove" that God(s) do not exist (impossibility of proving a negative), but that does not mean that the discussion/debate doesn't serve a purpose.
Many people in power make choices based on these beliefs, so it will never be useless to argue
Arguments involving God's existence exercise critical thinking, too.
It is not useless to people who benefit from believers such as pastors.
Nearly every argument proving and disproving God's existence can be countered.