The two main things that their deaths served as a catalyst for (Robb/Jeyne and Cat freeing Jaime) happened on their own, and they made a point that ravens aren't getting out. I don't see what else them finding about Bran and Rickon's deaths later on would add.
That being said though, I'm just disappointed because the major point of Bran and Rickon's "deaths" were what they kicked off. If Robb was going to sleep with Jeyne and Cat was going to free Jaime on their own time, there's really no point in faking the boys' deaths. Also it sort of implies that Robb and Catelyn were screwed from the beginning by the nature of their stupidity, which is wrong. They acted out of impulse and hbo really failed to show that, in my opinion.
Edited by Bride of Winter, 21 May 2012 - 04:51 PM.