From your other posts I gather you are young and strong. In my 20's I did not get too worried about weight of my pack. We older folks are only still going because we have really lightend up! But at any age and condition, a lighter pack makes the backpacking more comfortable. This assumes that the lighter pack as a suitable suspension system for the weight carried. It seems to me that the issue is not the pack, but what you feel you need to carry on a day hike. So, you may want to just borrow a small day pack from a buddy and see if you can cut down on your day-hike gear to fit the smaller pack.

For example, on a day hike either carry enough water or if there are water sources, take a filter and pump at rest stops. Do not to both. Even if you were to drink bad water, you will not get sick on that day. So it is not a survival situation. If you carry water, and spill it, or use it up faster than anticipated, then just drink out of the stream. Chances are you are not going to get Giradia. Really, how often do you think your water bottle will burst?

A day pack in the 1-pound range is really nice to have. You should be able to keep your day-gear to about 10 pounds - maybe 15 max.

There are lots of good day packs on sale for $30-$100 range. I have a small REI day pack (under 1 pound) that I even take when I use my Kelty frame pack on very long trips. If I want do some day-hiking from my base camp or climb a peak, I use it. Otherwise, it doubles for my sleeping bag stuff sack.

I do not think you will be sorry to get a specific day-pack.