Reason being, I prefer to have solid plugs that can be prepped and painted properly before moulding. You can get away with having plastic or wax or clay or whatever works on your plug, but it won't give you as nice a finish on your mould and so you will be chasing your tail trying to fix it. Of course if this is purely for weight saving and you don't care about cosmetics at all then that doesn't matter, but I always prefer to have the best of both worlds.