Ever since Midwest gave up flying directly to San Diego, I've been rather ambivalent about trying to fly them, and in fact have reduced my mileage accrual in favor of other larger airlines and networks. When I did fly them, I accrued Delta miles, figuring that once everything stabilizes I could go back to Midwest in earnest.
Sadly, it seems like all of this is continuing to spiral downward with the announcement today that the Midwest/Delta partnership is now ending.
If you want access to the Delta network for flying, you better book your reward travel now. It all ceases on June 7th. No doubt more changes will be forthcoming.