Load Lifters and some sort of support frame are needed with most packs. For pack loads of less than 10 pounds, you can probably skip them, since the pack itself, when compressed supplies enough support. Load lifters are not needed unless the loading exceeds the height of a frame used to support it.
The stiffness of the frames makes a huge difference in the performance of a pack. For anything up to about 25#, a fan folded pad works well as an internal frame. For loads up to about 35 pounds, a stiffer frame is wanted. Plastic or metal frame sheets, stays, etc can all be used depending on the load to be supported. For heavier loads an external frame is likely wanted.
Most lightweight, UL hikers or through hikers rarely reach 35 pounds. The exception is during desert hiking where large amounts of water are needed. Each gallon(US) weighs about 8-1/4 pounds or so. It is relatively easy to have a low base weight, fuel and food of about 20 pounds and still carry a 45pound pack. An external frame makes sense for these conditions. More normally, water is not a big weight, though. I typically hike through woodlands carrying about 1liter of drinking water. All else is made up as I go at breakfast and supper. I think this is a little low for many, but even two liters would be plenty for most hikers on a day’s hike...about 5 pounds.
Anyway, all packs can benefit from some sort of frame. A light internal frame, such as a frame sheet or pad, works well for light loads. A heavier frame sheet, thicker/stiffer pad, or, stays works better for mid range loads. External frames work better for heavier loads. What is *not* explained when you buy a frameless pack, is this does not save weight in and of itself. Rather, it saves weight *indirectly* by allowing the dual use of a sleeping pad (in whatever arrangement, tube, fan-folded, or structured) as a frame for lighter loads, ie, usually up to 20-25 pounds.
My conclusion is that load lifters are still needed on “frameless” packs for loads of ~20 pounds, because any experienced hiker knows he needs the support of some sort of internal frame. It really doesn’t matter that a pad is used or a “dead weight” frame sheet. The hiker will most likely use some sort of internal frame (or in the case of Gossamer Gear or other packs with pad pockets, a light duty “external” frame.) Load lifters, will help stabilize the load against collapse.
In the case where the shoulder straps come even with the frame, pad pockets or sleeping pad, clearly some compression of the pack will help to maintain stiffness against both front and back collapse as shown.