I'm sure Sharn was hoping for a more long term advantage from the moral dilemma task than what she actually got. It must have been a relative disappointment. It didn't make much sense either, if someone is being chosen to be sent there it's unlikely to be a person who is in trouble and needing immunity desperately at that moment. So I can't really fault Sharn in expecting something different.
However her explanation of the alternative (biscuits to be shared with another) sounded way too much like what Mat suggested before she went away , I think she needed to think up something more original.
Most of them seem to trust her anyway and I don't know why. Obviously a barrister should be good at convincing people and that's not a person you would send to that task.