The original levers always felt like they might snap if you squeezed them too hard. I once broke one by (unintentionally) hitting a car mirror; the mirror survived, the brompton lever snapped.
I have nearly had serious accidents on a brompton, braking hard at speed and having the rear triangle start to fold up. If you are going to ride a brompton with effort, brake hard or go through big potholes, I think some sort of clip would be very worth while.