I think Jackson's stated logic behind that decision that it rather undermines the ring's supposed power over the minds of men if Faramir can just shrug it off. It also makes Boromir look especially weak for almost instantly being seduced in Rivendale. Indeed, I felt that in the book, Boromir comes of as a complete oaf. Still, it's worth noting that in the film Faramir did eventually let Frodo and the ring go. I'm sure part of that had to do with padding out the Frodo/Sam/Gollum story to make the inter-cutting work better. After all, any good adaptation much be true to it's own medium and the book as written just wouldn't work if they filmed it verbatim. Plus, as much as I enjoy his works, Tolkien wasn't terribly adept at human drama, characterization or natural dialogue. Nor, to be fair, did he pretend to be.