Well it's up to users to make up there own decisions about FW updates. I tend to agree with your friend, if it works, don't fix it. However I to eventually update FW. It's nice that we can downgrade too on most routers. Some we can't. Users needs to review the release notes and as I recommend to everyone, if there not effected by any of the issues in the release notes, and there routers are working, FW updates may not be needed. Ya, problems need to be troubleshot to narrow down the issue and see if a FW update is really needed. In some cases it's not or the FW update doesn't fix anything.
As far as I know, the 645 may not see another FW released unless there is a security issue. I am wondering why I see a B12 on a EU site though. No release notes and it's not officially released.