Depends on what you're playing. If you're just playing through the game then it's definitely useful, but not necessarily if you're playing competitively.
When it comes to battling Rest, Sleep Talk and Curse are all common non-offensive moves. You generally want to have some kind of synergy between your different moves and the three ones I just mentioned are all good on Snorlax. You could give your Snorlax Choice Band if you really only want offensive moves, but otherwise I would use Curse or Rest/Sleep Talk.