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.
We don't know what Sam needs to do with Ruby in order to increase his power, but we do know that he got quite the jumpstart when they slept together during Dean's tenure in hell. The subtext in "Criss Angel" strongly suggested that Sam would be sleeping with Ruby again. That's not the only interpretation, of course, and it could be the usual misdirection. But if it were true, the writers could be setting up Ruby as a kind of succubus. After all, we haven't really seen what Ruby is getting out of her alliance with Sam, other than the vague motivation of setting him up as the Antichrist Superstar. In other words, the writers might be begging off a one-off succubus episode because they've been setting one up for a long-term arc instead.
Sam's point about Dean makes complete sense from his perspective, but it's hard to blame Dean for wallowing in a bit of self-loathing. It is a fitting follow-up to Dean's psychological state in "After School Special", however, and hits at the heart of his poorly hidden self-esteem issues. Dean had something to prove even before his soul was tarnished. How that will translate into the future remains to be seen.
I must give kudos to the writers for pulling a very nice bit of misdirection. Having set up the siren as a femme fatale, it was an easy assumption (based on the parallels) that the party girl doctor was the culprit. At the most, I thought the FBI agent might be partners with the siren. The writers stretched the siren concept in some interesting ways, and they deserve credit for a job well done!