John was not wavering in his message, as a reed swayed by the wind:
Verse 7. [...]
A reed shaken with the wind?] An emblem of an irresolute and unsteady mind, which believes and speaks one thing to-day, and another to-morrow. Christ asks these Jews if they had ever found any thing in John like this; was he not ever steady and uniform in the testimony he bore to me? The first excellency which Christ notices in John was his steadiness; convinced once of the truth, he continued to believe and assert it. This is essentially necessary to every preacher, and to every private Christian. He who changes about from opinion to opinion, and from one sect or party to another, is never to be depended on; there is much reason to believe that such a person is either mentally weak, or has never been rationally and divinely convinced of the truth.
Commentary on the New Testament. Volume 1. Adam Clarke. London, 1817.
Adam Clarke goes on to say John's clothing was another excellency. His simple clothing was a constant reminder that John did not try to ally himself with the world. He did not try to augment his preaching by wearing fine clothes in order to help sway public opinion in his favor. John was not worldly-minded, which showed most clearly in his choice of simple clothing.
Verse 8. [...] Let it be well observed, that the preacher who conforms to the world in his clothing, is never in his element but when he is frequenting the houses and tables of the rich and great.