Re: IDs in the URL (which I think has already been addressed)
What's to stop two people from creating threads with nearly identical titles? Especially something like "HELP"
and the more urgent "HELP!!!"
The title alone isn't unique. The ID is a very easy and low-impact solution: stick it somewhere in the URL so that you can pull it out and make the rest look as pretty as you want.
(meanwhile the forum name is probably unique enough to be by itself, but there's something to be said about the consistency of "name-id" for both the forum and the thread)
Re: Friendly URLs
They're one more thing search engines can look at. Certainly relevant, though I don't think they matter as much as some people think they do. It's not like changing the structure of your URLs will suddenly bump you up in the rankings. Page content matters a helluva lot more than the URL (which the prevalence and effectiveness of phishing demonstrates). "Oh but that's what the user sees, not the search engine!" The whole point of a search engine is to get people to the pages they want to see when they don't actually know where it is. They have to rank pages according to how real people see them; if they ranked otherwise then they would be pointless because they'd be so ineffective that nobody would use them. Quite logically, the most effective search engine will be one that sees pages the same way users do. Blah blah blah.