"If you accept that it makes sense to allocate on rename commits for overwrites of *existing* files, it follows that it makes sense to commit on *all* renames."
Renaming a new file over an existing one carries the risk of destroying *old* data. If I create a new file and don't rename it to anything, it's possible I will lose *the new file only*, on any filesystem (unless I fsync()). This is universally considered an acceptable risk: losing up to a couple of minutes' work (but nothing earlier) in the event of a system crash. This is the exact risk carried by renaming a file to a name that doesn't exist -- unless you gratuitously delete the old file first, which is completely pointless on Unix and obviously destroys any hope of atomicity (if the system crashes/app dies/etc. between delete and rename).
"Only files for which atomicty matters are renamed that way -- which are precisely the files that would get the commit-on-rename treatment in other circumstances."
Virtually all users of this atomicity technique appear to rename over the existing file, which is why almost all problems disappeared when users applied Ted's patches. Gaim only did otherwise as a flawed attempt to work around a quirk of the Windows API, in a way that wasn't atomic anyway, and that can be expected to be fixed in Gaim.