If you remember, it was not their initial decision to pass through Moria. After deciding not to go through the Gap of Rohan, they decided to take a path through Carhadras. They were prevented from passing through because "the mountain didn't like them". The remaining option then, would be Moria.
I'm not too familiar with details in the movie, but I think in there, Saruman (through his servants) deliberately blocked the fellowship and forced them to take Moria, thinking that Gandalf would not be willing to take it because of the presence of the Balrog (and of course, Gimli liked the idea of going into the Dwarf-mines).
Besides, in terms of plot, that bout with the Balrog was just necessary to thicken the atmosphere of despair and suspense... to later usher in a bright, dramatic hope (TTT) in the return of Gandalf as the White Wizard.
__________________
The only thing necessary for the triumph of evil is for good men to do nothing.
|