A lot of successful forums get their traffic in other ways than high ranking on SE. Some are just very well known for their domain name, others do not have that advantage and need to focus on SEO.
We already had a successful forum running long before using friendly URLs, but there was a considerable traffic increase with the introduction of the static URLs. The biggest difference we saw, was in the ranking of pages. We got strikingly higher in the SERPs, and more pages got indexed when they transformed from dynamic to static.
I'm not saying it did wonders, but it was definitely better than before. Much more guests, more registrations also. On top of that, our members love the friendly URLs, let's not forget they are, well... more user friendly.
SEO'd URLs do not have to contain long thread titles though. You can have a thread URL like
http://xenforo.com/community/threads/2333/page-2 and it will still have a pretty good ranking. I'm just saying that it's best to stop using things like "?" "index.php" or "name.php?ID=796887&sort=260807" because most SE don't like this, and SEO is still an important aspect of creating traffic.
It's not about "getting" the URL, it's how they treat it afterwards. Dynamic URLs are not treated the same as static. This is not something that is made up, it is clearly stated by Google itself. You would know that, if you would have researched this a bit more.