Why didn't he just take it?

[Beanamir of Gondor]

I agree that the whole Faramir-taking-the-ring thing would definitely have forced the entire plotline to be rewritten from halfway through TTT all the way to the end of ROTK. No way would Phillipa Boyens have agreed to do that, or PJ have consented to allow it.
But my real point in posting is that I think Movie!Faramir was just shocked and whipped when Sam told him that Boromir tried to kill Frodo to get the Ring. I mean, honestly, if someone told me that my brother, my beloved brother and role model, had tried to kill somebody "after swearing an oath to protect him", I would stop in my tracks and rethink what I was doing. Faramir, in my opinion, realized just HOW powerful the Ring was, and possibly considered what the heck it would do to his dad, once he had his hands on it.