From my experience W/ frameless backpacks (only a little)I'd have to say internal frames are a MUST for any load over 20 lbs.
So-called "frameless" backpacks, even ones that utilize closed cell sleeping pads, usually end up sagging around one's hips after a few miles "shakedown" of equipment.
The very minimal weight of internal stays in a well designed pack pays off in miles of (relative) comfort compared to framless packs.
That said I'm sure someone will soon come along with inflatable tube "frames" and turn pack suspension in a new direction. It's already happened with tents.
Eric

