I understand that the SEO benefits of rewriting forum thread urls is very debatable - I was pointing out an unrelated benefit for real human people rather.
Very often Joe Public won't take your links and give them good context, they'll be copy-pasted asis. For people reading those links pasted to other sites or in a blog, before clicking on it, a showthread link gives me no idea where I'll end up other than the domain and "it's a forum".
A rewritten url offers extra human readable clues BEFORE BEING CLICKED through to the page to discover the pagetitle or content.
It also reassures the copy-paster that they've pasted the correct link.
I was merely pointing out that SEO isn't the be all and end all reasoning behind rewriting obscure thread id number urls, people-friendly urls are as important to me as search-engine-friendly