People have this idea that this feature works against them; and that bots take up that number. I see differently, I gauge a website's popularity by this "most users ever online" stat when I buy websites.
Its like for a halo niche website, I saw 1,000 people logged in at a single time, that tells me that the site was popular. That tells me that the site has a strong pull. I see the same thing for a networked forum... especially if that forum consistently brings in at least that much, and then some.
With xenForo, I don't know this. So, I never pay attention to any xenforo forums that have a lot of members and posts. That is, if I happen to be on the market for a large board with xenforo on it.
This is why I never launch my sites with any other free forums, because they don't record this stat.