I can see what you're saying in principle, but what if a bug is opened with "this stopped working for me" but evolves into a very specific issue? Should the maintainer open a new bug, link to it and close the old one?

In practice, I find it really useful to tidy up the top message of a bug report (but /really/ like seeing history of it). I also see cases where users spam raw logs into updates and tidying it up (perhaps making the log portion an attachment or changing formatting) makes it much clearer for future-you and other collaborators.