The biggest reason for the inbetweening IMO, is simply to allow you to add more waypoints after you've screwed up
Without the inbetweening you have to delete waypoints until you get to the point you want to change, or you'll end up changing all your waypoints up until that point to fix/change what you wanted to happen. Inbetweening, just like the name, gives you the ability to fix your mistakes on the fly. (lets add waypoints to our existing points!) It usually doesn't take long to get an idea of what you want an airgroup to do and soon find out after adding another 40 airgroups you need to change some stuff up.
The inbetweening is a great thing to have and a huge time saver. I've never thought of it as a speed fixer or a way to make gradual changes that the normal waypoints don't do, but perhaps it does that as well.