Yeah, I'm not fully understanding this thread. Why would I want to do that.
* You PS+'ed it, you forgot about it, you let your Plus lapse, and now you want to play it again but PS Store says you already have it even though your copy of it is expired.
* You PS+'ed it, it's a good offline multiplayer game, you again let your Plus dry up, and now a friend wants to play it with you but, again, PS Store says you can't have it.
* You had PS+ and you got one of your favorite games through it, but then you had money problems or your PS+ account got screwed over one month and it got canceled on you, and you lose access to your backlog...
(Nope, shouldn't be a problem. If you lose PS+ for a month but then resubscribe, everything on PS+ is accessible once again.
* You get the Plus version for PS4 (or Vita?), the PS5 version comes out, but you don't have the "Complete" edition and for whatever reason the upgrade path isn't there, and you want to get the upgraded copy, but it says you have it just not a version compatible to upgrade and so you're in next-gen limbo (this one is a stretch... but also, the version control on PSN and I assume XBL/Steam/whatever is a mess and I've almost bout DLC or stuff before that I already had)
Basically, if you give up on PS+ or you for some reason have some PS+ problem, you need access to the store to get what you want. Previously, Plus counted as a purchase, and you could only own one copy of a digital product per account, so if your copy was under a PS+ purchase stipulation, you might have some problem getting a "clean copy".
(I was confused at first why this was a problem, but I've had some PS+ frustrations that I pretty much would have thrown money at if it made my problem go away, so I'm starting to get it.)