I agree that the 717 would be an excellent plane for Shuttle by United, and any of the short-haul type operations like exist on the west coast. But considering that UA has an extensive fleet of 737s and is growing a fleet of A320s, I doubt they would want to add the 717 as well. I'm not certain, but I can't recall UA ever operating a DC-9 or MD-80 series airplane (maybe they did early on, I don't know). Airlines do sometimes make suprise moves, so you never know.
Also, using longer range aircraft like the 737-500 does make it easier to pull a faster turnaround, as they don't have to refuel at every stop, although this does mean that they're carrying around excess weight in fuel, something airlines generally don't like to do. But it's an option.