Summary - 1 Corinthians 14:20-25

N.T. Wright reads verse 20 as a statement of general principle. Paul is calling on the Corinthians to be wise - to figure out what should happen in a church service and why. They should grow up and discern the difference between babbling, indiscriminate use of their gift (Baker, p. 200) and when its judicious use is helpful to the church. Tongues may be a sign to unbelievers* and not needed by believers. They could drive away visitors who would see Christians as crazy and it could look like exhibitionism to other believers. Prophecy, however, was a sign which could be useful for building up both believers and non-believers. When prophecy revealed unknown secrets of the heart, listeners would marvel at God's presence in the service. They too would worship him.

Extra source: Cornerstone Biblical Commentary 1 Corinthians (William Baker), 2 Corinthians (Ralph Martin & Carl Toney), ed. by Philip Comfort. Carol Stream: Tyndale House Publishers, c2006.
*Since tongue speaking was also a characteristic of many cults and temples, the value of the sign could be debated.