My commute is under 4 city miles so a backpack is fine.
I carry my laptop and some clothes so weight is minimal. My North Face backpack also has one of those air channels built into the frame so i do get some level of ventilation between me and the pack.

Additionally how the pack is fitted to your body matters... not all backpacks are going to give you the same support and comfort level. You may even need a waist strap on your pack to offset the strain on your shoulders.



Distance and weight of what you carry are factors that need to be taken into consideration.