Initially the point was about roar, as the scarfer is definitely going to get locked, so you will have to switch either way. In this case, roar has no use.
When you are going to use it, the other pokemon should either get locked to a move that does no damage, so they switch and you force a second switch, or they use another move that can cause harm. If they don't switch, you'll have to endure the damage and then roar to switch them, which I find less productive, as you already have a similar move on Steelix.