In addition currently a user can delete or edit his messages.
Your argument could also be given about that.
maybe that looses information, but i know i found what i searched, and don't continue to read all threads. Which could be a lot of work, in crowded forum, especially for least forum active users.
But it is harder to disrupt a whole conversation through editing, and less likely then just changing a thread title(who are often used stupid, like Help in help forum that quite a classic)
And even when bookmarks get implemented, i would use it more for importance peace of information like BB post, then for common forum chat which i mostly search and when you are off some days it is already not so easy to find a thread in the help section cause it is so active.
Is it a "top-priority" feature?
No. Not even close to that.
But, this folder is for all suggestions level.
does mean no top priority, that you couldn't add pro and cons?