In general, people's contributions on the Stack Exchange network of sites should be judged on... well, the merits of each post.
While it's certainly a valid discussion, I believe that this would be, ultimately, a mistake.
Let me illustrate. I haven't passed the bar. (In fact, because of the split legal system in New South Wales, I may never need to. But that's a technicality, and I don't mean for it to detract from the crux of your suggestion.) Because of my limited legal education, I can answer certain types of questions and more readily cite authorities in support of those questions. At the end of the day, however, it's not my legal education but the quality of my answers that (I hope) people are voting on.
This is because even if I had been admitted, no lawyer is an expert in all fields of law. Even if we could designate certain specialists in some way, the fact that you are a specialist doesn't automatically make your answer superior to others for that reason alone.
What makes an answer better - in my opinion - is that it answers the question (you'd think this would be a given, but it really isn't), it cites appropriate authorities, and it is in plain enough language to be understood. The goal of this site is to make the internet a better place, and we can't do that if answers are privileged because of some appeal to authority.
Finally, a similar question has come up on Health SE - Should we have a “verified physician” tag next to certain users? - and I'm just going to quote Robert Cartaino's answer:
No, we don't sanction one user over another based on their credentials. That is simply not how the Stack Exchange model works.
We — as a community — vet answers. This type of certification of users had been requested as recently as our Engineering site (Are expertise verified user flairs possible?), but we simply do not pre-qualify one user's participation over another based on who they are.
A user's answers — or anything written on the internet for that matter — should never be taken verbatim. You should remain skeptical and vet the information you see. Ask questions and clarify posts wherever you are able. It is that strong culture of peer review and wiki-style editing that makes this all work. And that is the strength of Stack Exchange.
Another benefit of citing authorities - I hope - is that some people will take the time to try to read the relevant parts, and become more confident and able to do so. In that way, we're increasing access to the law, which is a particular problem. Note that this doesn't replace an actual legal education, and I wouldn't want people to decide they can represent themselves in court based solely on the fact that they've asked a bunch of questions on Law SE. However, hopefully this kind of site can help people be more informed about the law and the way it affects them, as well as their own responsibilities and rights with respect to each other. But this paragraph is my own sort of... aspiration for the site and if it happens is a good thing, and if it doesn't... well, the site can still succeed.