I suspect it may just be because those in charge enjoy having naked women on the show. Bathroom scenes have always been a staple of horror, haven't they? (Or have they? I have to admit that I don't watch a lot of horror.)

And Sam's got some way to go before he beats his brother on 'number of deaths'. (Seriously, Supernatural, how many of your recurring characters haven't died at some point? I think it's down to Jo, Bobby and Ellen.)