For me it's hard to remember why it has been coded that way initially and working since ancient times :) I see now there are perhaps 2 points on keeping the forum ID in the URL:
1) It allows to make well-structured forums hierarchy specially when it's generated using .htaccess (like here on our forums, you may notice that each topic is held like under its own sub-folder of the forum) - more related keywords in the URL could make the topic's URL more appealing to a search engine.
2) When topic is moved to another forum, it's URL changes and it's a signal to the search engines that something has been changed to the topic (when it's moved to another forum, this also affects the indexed content of a forum in whole). If there would be just a topic ID supplied in the URL, there would be not possible to catch the point if a topic has been moved or not.
I understand that the topic also could run without forum ID as its effect on even the 2 points above is not that powerful. But I doubt that I'd change this approach now. It affects nothing in the same manner as it would affect something important.