As a general rule of thumb, if there’s any chance your head could hit any part of the roll cage in a crash you need a helmet with that cage for it to be much use to you. And remember if you’re not in a harness and your car ends up shiny side down you’re going to move a lot further out of your seat than you can replicate by just sitting in the seat.
Cages are VERY beneficial on the track. On the street, with no helmet and a regular shoulder and lap belt arrangement they aren’t nearly as useful. If you smash your head against the roll cage the rest of the compartment retaining its basic shape doesn’t help you much.
