Dragging keywords to another level of hierarchie is similar to renaming, so you get the same warning as you would get, when you rename a keyword.
I found two reasons that a warning might be useful:
- If an image already has keywords, this might be, because you imported images tagged by another application, and you are warned, that the keyword name change might result in inconsistencies with your other source of keywords. So your are asked to reconsider if you really want to do it, just in case you did not know that the keyword is used.
- Renaming might merge keywords, and they are no longer distinctive.
But renaming will (should) not corrupt your Library ...
It would be nice to be able to disable this warning, since it always is extra work to didmiss this warning
I noticed that myself, as I'm just staring to use Aperture and Keywords, especally within Groups.
It doesn't seem to cause any issues, at least not that I've found and I've done a lot of moving around. Mostly to test out how Keywords work and to see what happens when I move them.
My guess is that it's probably letting me know I'm changing the grouping of Keywords, which does have an affect. If you have the word Portrait under Photos and then move Portrait out from under Photos, you'll never see Portrait again if you search Photos. While that may be obvious to some, others might not realize this the first time they make the move and then suddenly wonder what happened to Portrait the next time they search Photos.
This is what I have been testing to make sure I understand how Keywords and Groups work. Outside of what I outlined above, I've not run into any issues. However, there are several other Aperture experts who will most likely join in to offer a more comprehensive explanation.