Depends on the "summer"! In the Rockies, it can freeze and snow any month of the year. For example, five years ago, in Wyoming's Wind Rivers, for two Saturdays in a row it snowed (almost a foot was reported in Titcomb Basin) and the temperature went down to 15*F (20*F at Elkhart Park). (If my dog hadn't gotten sick, I would have been at 11,000 feet for the second storm.) I would not go out in either the Rockies or Cascades with anything less warm than a 20* bag, plus plenty of warm clothing.

Of course if you live in Ohio or Alabama....

In other words, your "summer" sleeping bag needs to be suited to where you are backpacking.