MikeJones If you index those parts of the forum, you have to ensure that those pages are usefull and rich content or you can be penalized by Google. With the actual SEO state of Flarum, the best advice is not index them.
Imagine the situation in which you have 30000 users in your forum, then Google will index 30000 pages which content is mostly unusefull and can be categorized by Google as "thin content". So... you can guess that your forum will be penalized soon or later.
In short, the reason is SEO.