It's totally wrong to say the only flyers that would be going between hubs are double connecting. It isn't just for people who need to fly 'ELM-DTW-MSP-ISN'. If you are going to ISN from DTW, you are taking that hub-to-hub flight on a single connection DTW-MSP-ISN.

There are LOTS of other reasons to have extra capacity between hubs. You have a lot of loyal flyers at your hubs. If one of them wants to buy an expensive ticket to say PVG from MSP, you dang sure don't want to limit their options by not having seats on MSP-DTW. You never want completely sold out hub-to-hub flights until the day of travel, it limits what you can sell significantly.

There is the additional benefit of irregular operations and giving options to stranded flyers to get to another hub. I know it seems like airlines care less and less about this, but certainly it factors in.