I think the idea is not bad. It would allow us to test the typical usage of Bitcointalk, e.g. if the typical post structure breaks something. So I would use it not so much as a showcase for new users, but to progressively eliminate the problems mentioned by suchmoon. And yes, it would be nice to simply see the forum in a new layout.

But there are two challenges with a Bitcointalk clone:

1) Duplicate content. If the site is allowed to appear in search engines, it could affect the ranking of the original Bitcointalk, as well if create confusion - for example, some threads could be indexed for "Original BTCtalk", and some for "Beta Bitcointalk". This can of course be solved with the "noindex" tag. This would however prevent people finding the site. But for the usage I mentioned first it would be the best solution.

2) Copyrights. In theory, every forum user is an author who can decide where their posts should appear. I'm not an expert but this probably would't be a problem if the "Beta Bitcointalk" appears on the bitcointalk.org domain. Otherwise, maybe something similar to the Internet Archive regulation could also apply. If this was a problem anyway, then the current staff could update the Terms of Use accordingly, so people writing in the forum would also give automatically permission for its posts to be published on Beta.

So I think both challenges can be solved, however it remains to be evaluated if the effort is worth it. Maybe also a part of the forum and not all could be scaped, e.g. the "important" sections like Development & Technical Discussion (I'm mentioning this also because of possible challenges with inline code) and some Local boards, while the "heavy" sections with lower quality content could be left out.