When a car is cruising on the street it is not really easy to tell if they have made serious mods to it or not. When online on a forum like this you can see exactly what mods they have done etc, if the plates are displayed and the theif is interested in that car because of it's mods it is easier to track down, rather then if the plates are not displayed, so it is harder to identify an indivudual vehicle.
I don't see a problem with people blanking out plates, it does no harm, and if the owner feels more secure that way so be it.
If you don' wanna blank out plates then thats fine too...