Yields don't matter much. In a depression, people's borrowing or lending/investing behaviour will not be effected by a difference of a couple percentage points. Their behaviour is entirely controlled by fear.
The purpose of quantitative easing, it seems to me, is to create large amounts of new demand out of nothing (in other words replace the demand that is lost by depression psychology), thus putting idle capacity to work. Interest rates have little to do with this.
Changes in yields reflect people's willingness to save, rather than cause them. So I think Sam's point is rather the old monetarist interpretation of Lincoln: you can fool all of the people some of the time and some of the people all of the time but not all of the people all of the time.
