Part of the point of these methods of attracting ants is that the plant feeds the ants just enough to keep them around. The ants can easily stock pile a ton of food, but know there will be more in hours or a day.
By rationing the plant encourages the ants to hang around and defend the tree as their territory.
Aphids are doing something similar.
I wonder if bees ever also help flowering plants by scaring herbivores? Pollination is great, but could there be more to the story?