There is nothing wrong at having a guest speaker come and speak at our churches, as long as they are not disrespectful to the preacher/minister of your church.
Almost every Baptist church I have been in or gone to as done this showing their unity among one another........
We just had a guest preacher come in and give the sermon this Sunday because our regular preacher was sick.
His message was about our responsibilities as believers in Christ, and why teachings exist now saying nothing is required of us when our Lord Jesus said, "To much has been given, much will be required."