I did some research on lilies (Asiatic and Oriental) for an article I
wrote last year and the expert I consulted also recommended deep planting,
in part because the deeper the cooler, and in part because the deeper, the
better the root system and the sturdier the stem which, especially with
lilies, tends to be a weak point. I am sure you've seen lilies splayed
out on the ground with their over heavy blooms just weighing down the poor
stems. Supposedly planting bulbs deeper is a way to counter this phenomenon.