Astronomy said it better than I could (surprised face).
Ventilation is #1 because, as Astronomy stated, the pack rarely comes off during the day and I hate prickly heat soooooooooo bad.
The other reason to use a frame with the small load is to have the ability to take on extra weight (most likely water and batteries on the way in, but also SSE material on the way out) temporarily without having to change everything. Similar in concept to an elk hunter carrying a small amount of supplies for a day, but packing out meat if he's successful.
For hikers, Lowe Alpine and Deutor both make packs with some standoff from your back that work pretty well in the humid southeastern states. Worth looking into.