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.

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.

It is not useless to people who benefit from believers such as pastors.

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.