When I first learned git, I had no idea what staging meant and the differences between A, M, D, R, AM, etc...
Only some time later (albeit a short time) I learned about -a, at which point I had a bit more understanding of the statuses and what it meant to stage a change. If you gave me -a before that, I would have never understood those things properly.
How are you going to keep them from learning about -a?