In my case I assume it was a sync issue between browsers on multiple systems, but since it was a folder of sites I only needed to access every few months it left me with a big window for when the loss occurred. I could have downloaded all of the bookmark sets to be able to search (or otherwise track changes) but that's a one-at-a-time process through a clunky web interface. Last time I looked there was no way to search through the old bookmark sets, nor is there any kind of automated changelog or indication of what changed - not even a count of number of bookmarks in each saved backup. Even having a count of the # of bookmarks would have helped, because I could have looked for spots where the total number declined since I'm bad about doing cleanup.
Overall I'd call Xmarks "just good enough to keep me from actually deciding to try to roll my own solution" which is really a pretty low bar since I have some idea of the development scope I'd be facing.