It seems that a lot of consumers have this question and it is one that is very interesting. Probably the main reason why someone would want to leave their meat in the packaging (whether that be in a plastic bag or in the shrink wrap it came in) is to avoid any condensation buildup on the board and possibility on your kitchen counters. Some buyers have claimed that they have left their meat in plastic wraps and they have thawed just fine on defrosting trays.
Yet, in order to maximize the contact area, you may want to take it out of the packaging. If you are worried about the meat being exposed to the surface, the surface of a defrosting tray should be specially designed to be food-safe and non-toxic. So, there really is no reason for this to be a concern.