I bike commute with a Dakine Apex backpack, that has just about the best lumbar straps made. They make an enormous difference when cycling with heavy loads. They allow me to loosen up on the shoulder straps, lowering the pack down closer to my waste, thus relieving me of undue upper back strain. I honestly can't imagine not using the lumbar straps with a load greater than maybe 8 or 10 pounds.

For really light loads, the Apex allows me to remove the straps by simply detaching the heavy duty velcro. Very easy to add and remove, and easy to stow away in the hydration slot.

For the record, I also use the sternum straps when cycling. The more straps the better. I don't like the load shifting.