Even though I too have learned that the trigger area (TA) catches lemmings in the ground, not on the ground, it costs me mental energy every time to apply this rule. Either NL remains harder to understand for new players, or existing players have to re-learn.
The symptom is that NL culture suggests a shifted trigger area (TA) that doesn't overlap water exactly, but instead suggests that the TA be shifted down by 1 pixel. This catches walkers on the bottom of a water pit. Indeed, the culture tells you to mark nonexistant void outside of the tile as trigger area!
From IRC yesterday night: Nepster considers purely visual fix in clear-physics, editor, and graphic set tool. That means that what the programs display is shifted from what is written in the tile files? That can easily count as back-compat, because given powerful tools, you don't have to textedit the raw tile files. It would be offensive to interpret numbers in the same data field differently all of a sudden without a deprecation period and a field rename.
I see the main benefit for tile designers. Gronkling made perfectly rectangular water; a natural-looking TA would cover this exact rectangle, and no more. The proposed change, even if purely visual in all tools, allows that natural-looking TA.
-- Simon