It re-adds the books to the database, but doesn't re-create any missing metadata. That means all your tags, corrections, etc. won't be re-created unless the metadata in the saved books had been updated. That generally happens when the books are loaded onto a reader, the format is shifted, or the books are Saved to Disk. If all you want are the books themselves, then yes, importing from the old Calibre library will pull everything in, but possibly with stale metadata.
Well, that's just my opinion. My thinking was that the week old database still has up-to-date metadata on the books you haven't added and it may be less work to re-create the metadata for 100 books rather than more. You may feel otherwise
Corruption can happen at any time, but I haven't noticed that it's common, especially considering how many people appear to be using it. Of course, I haven't been paying any particular attention so my views my be completely wrong
I didn't mean to imply that anything's wrong with your system. I may have simply misread your posts. When I read your multiple posts, I seemed to pick up some frustration and assumed that this had happened more that once. If so, I thought maybe there's something else wrong with your system, such as at the file system level or maybe a hard drive starting to go bad. Again, I didn't mean to imply anything in particular and may simply have misread your posts. If my posts implied anything else, it was completely unintentional.