"Who cares if the pack weighs 2 more lbs if it will make the pack feel 10 lbs lighter and have less effect on your body?"
A pack with a more robust frame might 'feel' lighter to some select tissues in your hips and shoulders, but the additional weight of that pack still has a negative contribution to your overall fatigue level, so you need to balance these two. A 2 lbs heavier pack is 2 lbs heavier to your legs that have to carry it every step of the way. I'm not saying get an uncomfortable pack, just that you need to think about the entire picture. Your hips and shoulders might be a bit happier with a more robust frame, but your leg muscles, knees, ankles etc all need to carry that extra weight too, so there is a very real downside. This toll isn't as obvious as sore shoulders because it's just 'overall fatigue' but it does make a real difference. IMO, get the lightest pack that is still sufficiently comfortable to your shoulders and hips.
One mistake that seems to happen a lot is that people try one UL frameless pack and then they decide it's not for them and they go back to using a 3-5 lbs knapsack that supposedly feels lighter than that 1 lbs knapsack ever did. A lot of people seem to miss the middle-ground where you get a great pack with stays or even a light internal frame for 1.5 to 2.5 lbs. Something like the ULA Circuit (2.25 lbs) is likely the heaviest pack I'd ever consider. IMO, pretty much anyone can find comfort in the 1.5 - 2.5 lbs range even at 40 lbs. I can't think of any reason to ever start looking at 4 lbs packs unless we're planning a long winter mountaineering expedition.
For 3 season hiking, I can't think of too many scenario's where you'd ever be over 40 lbs. If you have a reasonable 8 lbs baseweight and you carry 1.4 lbs of food per day, you would only be at 26 lbs with 10 days of food and 2 quarts of water. At 40 lbs, you could pack 15 days of food and 3 quarts of water.