Well, its better to start doing things the right way first rather than falling into a habit
Ehh... Not necessarily. Again, its only necessary if you're getting into a physics based game, and if you're doing that you've got bigger fish to fry than worrying about moving your objects with velocity rather than position.
My economics professor has a saying, KISS. Keep It Simple Stupid. I think it especially suits programming
And its a fair assumption that he's using GameMaker. If he isn't he should just make sure to setup an xprevious var at the beginning of the game loop, easy