I do backup my purchases, However My preferred system is still to backup windows. Installing some of the Video programs I own is time consuming. Throw in the Windows Updates, and all the other software updates.....................
Then add in the programs that other programs depend on......................
The bigger flaw in depending on re-downloading and activating is what do You do when the Vendor is out of Business? Give up on a program you use and like? Or as You say Buy it again at a upgrade price and then find out that it has changed and You dislike some of the changes.
That explains why I have DVDs with the downloaded programs and the Activating key's email as well as a Loose-leaf binder with printed copies of the activation code.
As a point of information, Creating the backup on this computer takes less than two hours. I use two external backup drives. I alternate between the drives and I backup when I am not needing the computer. Then I can copy a backup to a NAS that has mirrored drives after any major changes. Such as if I were getting ready to upgrade to Windows 8.1 I would do a backup beforehand.