"I have one of the sternum strap whistles, and they're strictly the backup of a backup."

I would basically agree with your statement, but the sternum strap whistle is always there. I bought a new pack this week, and I noted that the sternum strap whistle was part of the package.

The whistle you have handy is better than the one back in the drawer. I typically carry at least one other (louder) whistle with me when outdoors.