We are not to regard anyone according to the flesh—that is, the outward person. Usefulness in the Kingdom of God is not determined by superior or inferior attributes.  

Paul admits that at one time he considered Jesus according to the flesh. He must have concluded that Jesus was inferior because Jesus did not have the training that others had. Jesus had been despised and rejected of men. There was no beauty about Him that would attract people. However, Paul later discovered how wrong he was.

Just as we are not to regard others according to their superior or inferior qualities, we are not to regard ourselves in terms of our outward attributes. Romans 12:3 warns us about such thinking.

For I say, through the grace given to me, to everyone who is among you, not to think of himself more highly than he ought to think, but to think soberly, as God has dealt to each one a measure of faith.

Consider also that Paul reminded the Corinthians that most of them were inferior to most people (1 Corinthians 1:26-28). Their need was not to think too lowly about themselves. 

After Paul made the statement in 2 Corinthians 5:16 about no longer regarding a person after the flesh, in verse 17 he writes:

 Therefore, if anyone is in Christ, he is a new creation, old things have passed away; behold, all things have become new.

The real issue in a believer’s usefulness in Kingdom work is not his natural characteristics. It is in his being a new creation. If a person is in Christ, Christ is in Him in the person of the Holy Spirit. Christ, then, can become his life.