I know it sounds weird but in my opinion we should remove the +2 rule at all! It should be solved or DNF only. Almost every +2 solve is because we rush the end of the solve to get a better time, ending up with a worse one. (That's why a +2 on 7x7x7 happens rarely and happens often on a magic.)
We should learn to solve the puzzles. Correct me if I'm wrong but for example in the final of the World Championship there were no or only a few +2-s, because the competitors focused on their solves enough to avoid this penalty case.
Maybe you haven't had this problem, but +2s are not all cases of "I will not bother fixing the last side, because it is just 2 seconds". I've seen and experienced cases where the cube was solved when the competitor dropped it but ended up a move off for some other reason; for example, maybe the cube is just too loose, or maybe it fell off the table or something and misaligned. It is very unfair to completely disqualify a solve just because of a mistake that may not have even been the competitor's fault.
As for the cube just turning: I have this a lot on 2x2 (Eastsheen cubes suck) and the only possible solution would be to spend an extra 0.5s carefully placing the cube down. But this is not a viable solution because 0.5s is a lot on 2x2 - it can be the difference between 3rd and 6th or 7th place. I know this problem does not come up all that often at the highest level, but it definitely does come up, and I think that giving a huge penalty (like DNF) for bad luck will just add frustration, make people slower, and make luck more of an issue as opposed to skill.
The thing is that we do not have advanced enough equipment to check the state of the cube
as it leaves a competitor's hands. If we did then we would not need the +2 rule because a competitor who really did solve the cube would always be considered to have solved it. Because of a bouncy surface and some competitors having loose cubes, there are always going to be misalignments that happen completely by accident, and declaring that that should be a DNF is ridiculous.