I often use my WM bags as quilts (as a "throw" really - I unzip the bag and use it as a blanket). The lighter weight bags (Summerlite, Megalite) don't have enough down to fill out the bag to use as a quilt. So, I have mine overstuffed by WM ($10/once). I added 3 oz to each. The extra weight is worth it for me to have the added ability to use it as a quilt (which broadens the temperature range for which I can use the bag). When not overstuffed (and used as a throw) they have the same problem you described - migrating down.

Just to be clear - when used as a sleeping bag: when the down is shaken to the top of the bag it has a tendency to stay on top because it has no where to go as you are sleeping on the other half of the bag. So, the lighter bags work well as sleeping bags, but not as a blanket (Unless fill is added).