So, I see this quite useful if the guest wants to then become a user. That is, sort of a "try before you buy" mentality. A guest might enjoy the experience and then decide they want to sign up. So, they could reupload images to their new user account, or migrate their guest images (with all their stats and such) to their new user account.
I'd see this as a nice feature to have, but wouldn't prioritize it too heavily, imho.