Generally speaking, you only need the most recent catalogue and preview data.
What sense would it make to have a two month old catalogue.
I always hand copy my catalogue and the preview data to the external hard drive.
Now I'm getting another external hard drive for double back-up (just in case something happens during the back-up process).
But I understand your feelings. It is always weird to erase back-ups. I haven't done it myself. Possibly digital photographic superstition.