As Brogan said, it is not necessary fro xenforo to work. But it can be useful for SEO as (theoretically) it would stop Google from indexing unnecessary pages that are either not useful in searches or could possibly (according to some experts) even be harmful for SEO.
I want to say it was one person on this form that has 1+/m posts, and he said he does not use robots.txt...
I would not go so far to say it will be a negative impact; The impact of less traffic from blocked pages...
Can you call that a negative impact? I have never had a site banned for having low quality pages...
some pages may not get indexed for obvious reasons, other then that, just dont expect to get higher
rankings that you may get from other indexed pages. Even low quality pages could bring in some traffic,
but you will certainly kill that off by blocking everthing from here to there. After all, if a million post
forum has no need for a robots.txt then that is good enough for me. Your robots file will not make your
site a success or failure, as your robots file does not determine your popularity....
The same can be said for VB SEO, everyone rushed out to get it because they felt it was a must have, and
if they did not get the addon, then they were doomed for failure. VB SEO did not determine their fate...
It was the forum owner that failed, having or not having vb seo had nothing to do with it, over the years
i have run across many successful forums that did not use any type of addons for seo urls.....