I've not really dealt with them as yet, hopefully won't have too unless it's to renew my plates!
If you turn the scenario around, and cover the car's plates with your own... and then get stopped for doing so. You'd be in a worse position, as you're clearly going directly against the official guidelines. I'd need to see some guidelines that clearly tell you to cover the plates (which is therefore contradictory) and then at least you can challenge them. It's pointless phoning the DVLA - if they tell me to cover the plates, it means nothing unless it's in writing.
To me it doesn't make any sense to cover the plates - if you do, you're obscuring ANPR capabilities for spotting stolen/marked/non-MOT vehicles.
Infact! If someone can point me to a document that says 'cover plates' then I will take both documents, and my plates, and raise the confusion with my local MP. See what he says.