Probably what they were saying is that you need to separate the hub assembly to fit the correct size backing plates.
But do you really need backing plates at all? Having run several cars with various degrees of brake upgrades I've only ever cut them off and not noticed any issues. I believe the purpose of them was that when driving on a motorway you can go for miles without touching your brakes, add in wet weather and your discs will be totally soaked including the pads, this will result in a second or 2 of poor or no braking (mainly on older cars, new cars have such good brakes you won't notice) the shield keeps most of the water off resulting in slightly more responsive brakes. You can just dab the brakes every so often to clear them in heavy rain, but on modern cars there is no point.
