Most members who have been banned usually demonstrate the same kind of behaviour which led them to being banned in the first place, in my experience. On that basis, I would never revoke a ban, unless it were found to have been issued in error, for example, catching an innocent party in an IP ban, which has happened back when IP bans used to be used more than they are now.
In addition, assuming your forum has clear rules and moderation procedures, the member concerned would, I assume, have had to have committed a significant number of breaches of the rules to be banned in the first place. On that basis, I would argue that reversing the ban would undermined this entire system, and result in other members of the community possibly, and justifiably, complaining about biased treatment of this individual. This would be another reason for not going down this path.