That's my concern too.
In the case of depressurisation, some passengers might be more conscious of the fact of wiping themselves and dressing up before rushing to the nearest O2 mask. Keeping in mind that most passengers are rather clueless how fast they can black out in an depressurasation situation.
Whether its tanks or pipeworks, the safety announcement basically meant that there is no O2 in the lavatory when there is a need for one and I am very sure the passengers in the seats nearest to the lavatory ain't going to share their masks.