Altar calls, when properly handled, are certainly effective. First of all, they remind the listeners that the gospel demands a response. As Billy Graham has said, "You cannot give God a definite maybe. It has to be a definite yes or a definite no." When the altar call is properly handled, lost people are asked to trust Christ as the only way to heaven. The issue is responding to Christ, not to you. The person therefore knows that to trust Him is to receive His free offer of eternal life, and to reject Christ is to reject that free offer.
In addition, when a person responds to an altar call, he or she is right there in front of you. Of all the invitation methods, this is the easiest way to get with the person one-on-one. You have not asked them to meet you in another room after the service, which they may not find, nor have you asked them to meet you at another time, allowing them to forget when. Instead, you have said, "Come see me, and come see me now." With them right before you, you can speak to them one-on-one, either immediately or after they are escorted to a side room.
A third advantage is what an altar call says to other listeners. As a lost person sees another walk forward indicating a need of Christ, he/she is tempted to think, "If that person is unashamed to admit his need, what's wrong with me?" The one responding encourages others to respond.
That said, there are situations and reasons where giving an altar call is not only wrong, it is dishonoring to God:
When it is made a condition of salvation
This first reason is the absolute worst. A television evangelist once proclaimed, "There are two conditions for salvation—one is to come to Christ, the other is to come forward." He continued to make it clear that, in his opinion, if one does not come forward, he/she cannot come to Christ.
May God have mercy on such a person—he has changed the terms of the gospel. Jesus so simply said, "Most assuredly, I say to you, he who believes in Me has everlasting life" (John 6:47). Not one word was said about walking forward through an altar call. Furthermore, if an altar call were essential to salvation, we would be confronted with two huge problems: For one, it means the thief on the cross, contrary to Christ's declaration, went to hell. The man did not and could not "go forward"; there on the cross, though, he acknowledged Christ to be the One He said He was. Jesus so lovingly assured him, "Today you will be with Me in paradise" (Luke 23:43).
A second problem comes up in John 12:42, where we are told, "Nevertheless even among the rulers many believed in Him, but because of the Pharisees they did not confess Him, lest they should be put out of the synagogue." "Believed in Him" is the Johannine phrase for "salvation" used throughout the Gospel of John. It's the same phrase used in John 3:16: "For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life." Other verses where this same phrase is used include John 3:18, 3:36, 5:24, 6:35, 6:40, and 6:47. Here were Jewish leaders who had sincerely trusted Christ, but they were afraid to confess Him lest they should be excommunicated from the synagogue. Such a verse makes it clear that trusting Christ, "believing in Him," is a separate issue from confessing Him publicly.