It's a spell only fitting for somebody like Harry.
Harry was against killing, and why kill when you can disarm?
And although Harry gave in a few times in the use of unforgivable curses (except for the Killing Curse), he only use Imperio because he needed to, and the cruciatus curse was only used when he was beyond berserk (for obvious reasons).
Yes, expelliarmus is a useful spell. You don't HAVE to kill or to hurt your opponent.
Caption this Meme
