In many situations, when I think about Tremolo, I think intensity. So in the places where you want to be intense, you can switch it to fast after being on slow for a while and it will create a dramatic change. The Chorus and Drawbars play a part in this as well.
So say the Pastor is just talking in a mild tone of voice that may be leading to a praise, you may want to keep it on chorale with the chorus off, and the drawbars on a mellow setting while the second set of drawbars is on full setting.
Now when he decides to go into a praise, you quickly flip the chorus on, change the preset key to the more full drawbar setting and hit the leslie switch, and that will make it so much more dramatic and exciting. You can even switch to a higher key while doing all that and it will make it even more dramatic.
So it's not just the leslie switch, it's how you use all of the other features of the organ together. There are times when you may not even touch the leslie switch for a whole song. See the below clip with Twinkie Clark. She doesn't touch the leslie at all, but instead uses the chorus and the drawbars to add dynamics. Also check out the other clip with PJ Morgan and he talks a little about that.
As for using slow during the verses and fast during the chorus, I see nothing wrong with that. It doesnt work for every song, but for me it goes back to the intensity thing. Sometimes the chorus gets more intense, so I may use it in that way.
