It might be a useful addition if, when a user with a tag badge answers a question tagged with their tag badge, that that information be displayed visually near their badge.
ie, when Jon Skeet answers a question about
C#, there will be a visual cue to indicate that he is, in fact, fairly well established in that topic.
That way, reputation information displayed near a user's info can be given domain-specific utility. ('Oh, this person has 100k rep but hasn't spent a lot of time on this topic' vs 'This person has 5k rep but seems specialized in this topic').
Obviously, we should be judging answers based on their accuracy and correctness, but the reputation gives us an indication as to how much 'benefit of the doubt' we should be giving on difficult-to-evaluate answers.
(There are two obvious ways to do this: The easier is to highlight their user info box in a different color to indicate the extra expertise; the other is to show the badge itself somehow, but that's a bigger change.)