Much like firearms, I never, as of this date. sold an old bow of mine. Got them back to the ones I had at 13 years old, Im 55 now.
Just this past month I have spent time rerigging my old PSE Greyghost with a new sight, interchangable quiver mounts with my primary bow and limb savers.
Like you said, you should always have a back up and I figure it's nice to have a variety of bows to shoot just to keep ones skills sharp.