The principles of an 'air-gap' are two: to prevent liquid from enter either through force or siphoning. By having approximately 1" space between the drain line and the drain no liquids should enter the drain lines of softeners or ROs from the drains 'flood level".
This is to prevent the drain lines from contamination which may find its way back to the equipment. Air gaps are generally to prevent liquid contamination (and the contaminants carried by liquids) but in some cases to prevent gaseous contamination, too. Backflow preventers can help prevent liquids from re-entering the equipment but may not do so well with gases. Gases are normally prevented with traps.
Your plumber is probably right when he says the chances of water entering at that height are virtually impossible (flood level being far below that point) but still not acceptable to codes and vents do nothing to prevent gases--in fact they are designed to contain and channel (sewage) gases.
This is to prevent the drain lines from contamination which may find its way back to the equipment. Air gaps are generally to prevent liquid contamination (and the contaminants carried by liquids) but in some cases to prevent gaseous contamination, too. Backflow preventers can help prevent liquids from re-entering the equipment but may not do so well with gases. Gases are normally prevented with traps.
Your plumber is probably right when he says the chances of water entering at that height are virtually impossible (flood level being far below that point) but still not acceptable to codes and vents do nothing to prevent gases--in fact they are designed to contain and channel (sewage) gases.