If you're going to backpack in the Rockies in summer, you really need a 20* bag. At high altitudes, clear nights are often frosty, and it can snow at any time. Last summer when I was in the Wind Rivers, it snowed down to 10,000 ft. on August 15. All the nights for the week after that were below freezing. My WM Ultralight Super (with me wearing a heavy base layer, balaclava and a vapor barrier inside) was a bit marginal a couple of those nights.

On the other hand, if I'd put the jacket on I could undoubtedly have withstood a colder night. I'm an unusually cold sleeper.

The continuous baffle bit may be fine for those who stay in one place for sleeping, but I do a lot of tossing and turning. As a result, I have to work at evening up the down along each baffle every night so I don't freeze when I turn over!

It's easy to use a warm bag as a quilt on warm nights, or just leave it open. However, we don't get the hot muggy nights out here in the Northwest that you get where you are, and during some of those you might drench the bag with sweat even if you just sleep on top of it. For your situation, I can't see any way out of having a light summer bag and a heavier shoulder season bag.