The raid/trap by the Orcs was perfect because the fog of war (along with the personal crisis of Frodo) created a situation of total chaos, where no one knew where anyone was. And so off they went in many directions. It's interesting that he thought of discarding Legolas and Gimli at this point (by sending them off back home) -- of all the Fellowship (maybe even including the now dead Boromir), from this point forward they have the least "book time". They play a significant role in the battle at Helm's Deep, but are riders-along for much of the rest of the tome. Perhaps he thought of "sending them home" because from that point forward in the story plan he didn't really have anything coming for them but bit parts -- and perhaps those bit parts were added to or embellished later on in the story to justify keeping them along.
I was just skimming the material and looking to see if he ever had a plot where someone goes after Frodo and Sam. No luck so far. I should add an in depth reading of "History of LOTR" just to make sure. :)
I was surprised that an early plot had Merry and Pippin wandering away and no one going after them (poor hobbitses!) Or was that the plot line where Aragorn thought all four hobbits were together? It gets confusing after a while!