Albert Barnes on 1 Corinthians 12:19

And if all were one member. If there were nothing but an eye, an ear, or a limb, there would be no body. The idea which this seems intended to illustrate is, that if there was not variety of talent and endowment in the church, the church could not itself exist. If, for example, there were nothing but apostles, or prophets, or teachers; if there were none but those who spoke with tongues or could interpret them, the church could not exist. A variety of talents and attainments in their proper places is as useful as are the various members of the human body.