It's when you start adding elements not in the original capture, It's no longer photography.
1. I'm rather late to this thread (but it hasn't reached the flamewar stage yet :-)), but I agree with the "adding elements" argument from RLPhoto.
The reverse means that any global adjustments
and even local adjustments via ACR
are ok, so if the sky from the original image of the op would have been overexposed it'd been ok to pull it down with a grad filter in LR.
One thing I'm note sure about is "repairing" small flaws in the background
with ACR, it really depends on how large the impact on the general picture is - and even small layer operations
like removing closed eyes of one person from a group shot with the equal part of another shot directly afterwards might be ok.
2. One argument I didn't find here is from an ad with Julia Roberts that was forced to be withdrawn because it was so heavily "beautified" that it wasn't Julia Roberts anymore, and I think in France it was feared that this is what shifts the view of what a woman is supposed to look like to an unhealthy stage.
Imho the same applies images: If everybody does it, even near-perfect "conventional" photos will look crappy
- so personally I never do masks in photoshop or add foreign elements, or I wouldn't consider it an old-school "photo" anymore.