As funny as an episode this was, it was sloppy and badly written.
Just adding onto the points that lilmisstxhas already mentioned:
1) The female witch who is suppose to be 100's of years old, very powerful and intelligent actually tells Sam and Dean what they did wrong with the spell that would kill her and her husband.
2) It wasn't made clear how the coins work or weather or not the guy witch could use his wife's coins but at the end when he electrocutes and knocks out the Leviathan, if it was without the aid of his wifes coin then why do they use the coins in the first place if they are so powerful?
3) Finally as lilmisstx mentioned, only a few episodes ago Dean killed Sam's childhood friend because she was a monster, a killer and it was his duty as a hunter to kill her.
Yet he doesn't even bother attempting to chill the chicken feet and re-attempt to kill the 2 murdering witches who regardless of the guy's good intentions at the end, he still killed an innocent woman just to annoy his wife.
I can understand to an extent why they didn't kill the guy witch off simply because he helped Sam and Dean capture a Leviathan (something they previously couldn't do) and perhaps he will be a future ally?
Either way, it's still sloppy writing.
I enjoyed the episode but i couldn't help but laugh at the inconsistencies in the story.
Personally i believe a good series should never go past 5 to 6 seasons to avoid getting repetitive, slow and stale and allow it to end while it's still in its prime.
So many series carry on because of their popularity when they shouldn't, i fear Supernatural has now become one of them.