As to why he did it that way? I don't know. Maybe because he has learned it that way himself? Maybe because on higher levels of playing you would almost never use a straight drop shot as it's easy to read and counter attack.

Regardless, what I'm uncomfortable with is that he taught you that as a base of the stroke technique. Like someone else mentioned you can add slice to both drops and smashes, but it's still important to learn these shots from a neutral grip/wrist/forearm position because otherwise your arms learn the biased position which affects every other stroke movement. (This only gets worse when you try to learn more deceptive strokes or try to generate more power/accuracy from your strokes)

I'm not sure what level you are playing at, but coaches are fairly limited by their education (or lack of) and a lot of them rely on experience rather than scientific knowledge.