This is a grave misunderstanding. In actuality, a stronger case is found for labeling "believers baptism" as useless. There, with God as a mere observer, the baptized party is either already saved or a counterfeit. In either case, nothing happens.
Does a believer's baptism serve as a public testimony? Perhaps it does, but not a very strong one. It is the day-to-day life of the professing Christian that bears witness to the genuineness of his faith, not the act of giving a public testimony and being immersed.
I feel very strong about that last point: Believer's baptism values as a public testimony is highly overrated, a red herring, at best a short-lived shot of adrenaline.
Infant baptism is a celebration of God's promise; it recognizes God's work, not the work of the person being baptized. Now there is a profession of sorts being made, not from the one being baptized but from the parents, and it is a profession of belief, belief in the new covenant promise. The parents are publicly professing:
God, you have promised me eternal life if I believe in the redeeming work of your Son. This baptism is a sign that Your promise extends to my children as well—that if they believe they too will be saved.
God's covenant of grace it its various administrations always had an associated sign of God's promise, not man's achievement. The promise is the same since the fall: the promise of salvation through grace. The signs have changed. With Abraham it was circumcision. With Noah, the rainbow. With Moses, the law. With the finished work of Christ, it is water baptism. In the face of this overwhelming continuity (not to mention scripture), there is no reason to convert the last and greatest sign of the promise of salvation into a celebration of something man does: credibly (yet still unreliably) proclaim his faith.
I find it interesting that in the covenantal view circumcision and baptism means exactly the same thing in all cases and to all parents: Thank you Lord for extended your promise to the next generation. Jacob’s and Esau’s circumcision meant exactly the same thing, even though one was loved by God and the other hated. Simon's and Lydia’s baptism meant the same thing even though (it would appear) one was counterfeit and the other genuine.