For quite some time now, Eric Kripke and the writers of "Supernatural" have stated that they've considered an episode featuring a succubus, but they simply couldn't find a way to make it work. This particular episode makes me wonder if that's truly the case. The siren in this episode may not be a succubus per se, but its methods aren't too far from that tree.
Just based on how the siren was depicted and how the mythology was treated, the writers were more focused on how to get the Brothers Winchester to start screaming at each other than the creature. Everything was built around the effect, which in turn was the culmination of the details that had been included in the past few episodes. Sam and Dean have been holding back on some serious angst, and the release was not pretty.
Dean's point is very well made, and it is the major area of concern at the moment. Sammy is on a slippery slope to the dark side, and Ruby's the one leading him by a leash. The writers definitely draw a parallel between the siren and Ruby, at least from Dean's perspective, and that was what reminded me of the succubus comment.