With frameless packs, I use a closed cell foam sleeping pad inside them, rolled up like a big burrito, in order to provide (what's essentially) the same structure that a light-frame pack provides. It's lighter than the light frame pack, though, because my gear is used for multiple purposes, rather than having a pound of single-use gear in pack frame.
Some people use frameless packs just on their shoulders, but I personally do not like carrying even 5-10 pound packs like this due to balance issues, and whenever possible, I always use a pack that can transfer some of the load onto my waist. Even my lightest backpack, a Zpacks Zilch, was ordered specially with a set of padded, winged hipbelt straps. With the winged hipbelt, and a rolled up ccf pad inside for rigidity, the pack provides essentially the same load transfer-ability as a traditional pack for very light loads (about 13-18 pounds max, depending on how it's packed, and which ccf pad I'm using inside it).
The basic principle of traditional backpack design is: keep the load balanced upright, above the load transfer point, which for a traditional backpack is the hipbelt. To keep the load balanced, the pack needs to be kept rigid with a frame of some sort (or else the entire pack can fold and lean away from the body and out of line with the vertical balance). The shoulder straps also work to keep the load above the hipbelt; basically they just keep the pack from falling away backwards. That's all there is to it. A pack frame suitable for 10 pound loads will deform when 30 pounds is put in it, resulting in uneven load transfer.
My Vapor Trail stays rigid up to about 30 pounds, but above that, the framesheet starts to fold in a way that takes the balance away from my back and it carries less comfortably. My Arc-teryx Bora 80 stays rigid no matter how much weight I put in it (what do you expect of a steel frame?), but then again that pack also weighs over 7 pounds...
Backpacking gear is all about compromises and understanding the results of those compromises. The frameless packs are lighter than light frame packs, but only workable up to perhaps 25 pounds, and only if you know how to pack them correctly. So for most backpackers, it would be a simple decision to choose the light frame pack. OTOH, if you can get the rest of your gear light enough and compact enough, and don't mind integrating a ccf sleeping pad (or a combination of ccf and short open cell foam (i.e. a 1/2 or 3/4 length theramrest), then the whole "compromise equation" is shifted more in favor of the frameless.