The Fellowship had to split up not only because they could not defeat Sauron by themselves but, Aragorn had to restore the Kingship of Gondor and had to lead the race of men agianst Sauron. It is also easier for two smaller people to sneak into Mordor than Seven more people of bigger size or equal size.
Boromirr was a big factor in the breaking of the Fellowship. If Faramirr went instead of Boromirr then the fellowship would have not been broken and might have failed. If Faramirr was part of the Fellowship he would have not captured Frodo, Sam, and Gollum. Frodo would not have had to "Trick" Gollum at the Forrbiden Pool and Gollum wouldn't have betrayed Sam and Frodo to Shelob. If Gollum did not betray the two hobbits to Shelob they would have been caught at the Tower of Cirith Ungol and would have failed.