Mean Streak has the top trims turned off every now and again but they were added for the same purpose as from what I've gathered they were added to Beast for: To keep the trains from destroying the structure.
The Mantis trims were different, they were there for the purpose of creating another block so the employees could have more time to put out another train. Mantis has only been running 2 trains since the 2000 season so those trims have been turned off.
I think it's a good idea to have magnetic breaks, despite the loss in speed, it's good to know they've got the well-being of the structure in a high priority.