Correct the lending library is only for devices, not the apps.
And I agree, the library and Prime Videos, which are the two biggest ancillary benefits beyond the shipping discount, would be hard to justify on their own.
I guess the exception would be if you look over the Prime Video selection and decide that you like so much of it, that it justifies it for you.
Or maybe if you had no other source of programming....no Cable, Netflix, Hulu, etc....
But I predict that works out for very few people.
For the vast majority, we'd need to make up the Prime cost with shipping savings, and the rest is just gravy.
FREE TRIALS of Prime, on the other hand: Yeah, baby!