It's true, in high temp environments (ISA+30), a 2.8 degree path will actually give you close to a 3 degree path
at those higher temps (the column of air expands with increasing temp). However, the problem is when the temp is low, it is back to a 2.8 degree path and if the ATIS QNH hasn't been updated for a while you can get on the slightly lower angle than the 2.8 (we only get 15 minutes from receipt of the ATIS for a valid QNH).
The RNP already has an allowed vertical error (ANP) of 125'. We also go to RNP 0.1 which can have minima around 370' / 2200m so you can be close to the hard stuff with some degree of uncertainty. If you then get a little low on that path...you are starting to get closer to the weeds than is comfortable.
So, rather than accept a G/S warning (and QAR flag) saying "Hey Gringo, you really are low", just turn the off the warning system and remove perfectly valid, usable, independent and rock stable path indication! Really, although not everyone follows the recommendation
