Whilst I think the reasoning behind the legislation is clear, I don't regard it as very sensible at all.
In the event of a decompression gasses are going to try and escape from the body as well as the cabin itself, restricting any orifice likely to assist this (e.g the nose and mouth with an oxygen mask) is therefore probably a very bad idea indeed - If you really want to rupture all of your internal organs then why not stick a cork in your backside at the same time as donning the mask?