In general, swept back bars don't benefit from the stem nearly as much as non-swept back bars or dropbars because of the single pivot suspension element. The stem absorbs shocks and vibration by rotating about its pivot point which is just in front of where the stem attaches to the steerer tube.  

Because it pivots, only loads that are in front of the pivot point will actually cause the stem to flex (a "moment" is required to make it move). With swept back bars that put your hands next to or behind the pivot point you will not be applying this "moment" because weight is not out in front of the pivot.

If you think your hands might be in front of the pivot point then feel free to give it a try and see if it works for you. But, if it is obvious that the hand position will be in line or behind the pivot on the stem then it will not be able to move and will feel like a rigid stem.