They don't show the same content, the commercials that the network affiliates sell are not and that's a source of revenue for the stations that would be lost. There's also the local weather warning crawls that wouldn't be possible.
a lot of stations don't have the abilty to do HD overlays so when a crawl starts they dump to SD. Note i said the national should be a SUPPLEMENT to the SD, they could make the national feed only available to markets that have SD feeds, then Dish could pay the network for the feed then the network could pay the stations