I think the vital problem with the Faramir-in-the-Fellowship argument is that it seems to stem from the films, in particular a flashback scene in TTT:EE. Correct me if I'm wrong, but that's the only place I'd ever find inspiration for such a scheme:
Boromir: My place is here with my people, not in Rivendell!
Faramir: If there is need to go to Rivendell, send me in his stead.
Denethor: Oh, yes. A chance for Faramir, captain of Gondor, to show his quality. I think not. I will send your brother--the one who will
not fail me.
Much as I love Jackson's addition in TTT, I consider it to be utter silliness, founded as much upon sentimentalism as the fact that Jackson wanted Faramir to be younger, more sensitive, and more prone to emotional action than in the book. (I always refer people back to
this article on Faramir for an amazing comparison.) The crucial question we're not asking is why Book!Faramir would ever have been sent to Rivendell in the first place. Boromir was the elder son, the favorite of his father, and when Gondor was summoned to the council, Boromir would have been Denethor's first choice, hands down.
Also, in terms of the film, I consider Faramir to have stood up to a LOT more temptation that Boromir: "Here I have a host of men at my call, and the Ring of Power within my grasp. A chance for Faramir, captain of Gondor, to show his quality." He realizes that his brother has totally failed in his mission to bring Denethor "a mighty gift", and feels a responsibility to do so. And yet he
doesn't take the Ring, knowing that he must resist
both his father's wrath, and the temptation of the Ring. If Movie!Faramir had indeed been in the Fellowship (which would mean that he had been granted trust by his father, and therefore would have been much more self-confident even than Boromir), I get this feeling that he would have taken on an Aragorn-like role: he would have fully recognized his temptation by the Ring, and in the same breath rejected it.
Let me conclude my lengthy and somewhat tangential rant by agreeing with what's been said: it'd be fun to argue whether or not the Fellowship would have survived if Faramir had replaced Boromir, but as
Mithalwen says, Boromir's betrayal of Frodo does, ultimately, serve simply as an irreplaceable plot device. You might as well as what would have happened if Sam hadn't agreed to go with Frodo!