Someone could argue that, after going through the temptation, taking Frodo to the gates of Osgiliath, and then redeeming himself by letting Frodo go, Faramir was made more wise and noble than he would have been if he had simply not taken Frodo prisoner in the first place. It shows that he can overcome the influence of the Ring even after it's had time to work on him, which demonstrates strength and resilience of character.