I think the idea was to give Frodo a threat. Remember that Shelob wasn't there to supply an awesome
cliffhanger (
) so the Director had to give Frodo some action. So he just warped Faramir into a baddie for most of the film.
Sad, I know; but Faramir's not the major character and is thus expendable (not that our major characters were immune from creative liberty but o well...)
My main problem with the Faramir-Osgiliath story is that it's boring and uninteresting. So-called 'action scenes' are not necessarily better than intense Frodo-Faramir interrogation (which would actually include acting).
Think about it this way: We already had the Battle at Helm's Deep; we didn't
need much more 'action'! Let's get some acting in the films; that's going to be much better than having some random Gondorian soldier shouting
"Look out!" shortly before a wall in Osgiliath collapses. (And how come all the action and noise stopped for a couple of seconds so that everyone could hear that guy's helpful remark? What was that all about?)
Free Faramir! Send Shelob back to
TTT!