Message customization does add to the game, however, it adds more then just cool messages. It adds bloat. This is one of the reasons we don't allow customization of NeXuS clothing, or armor. Because it just takes up so much space.
I think having a part add a message to the outside of a vehicle is viable, but that part would be a generic. Like chrome wheels. If you added chrome wheels, the message would be the same on every object.
That would allow some customization, without having every vehicle defining several custom made messages by the mechanic.
However, I'm open to discussing this. Though, I think Johnny will bring up the bloat argument as well.
