Essential as in the first and so far only permanent manned orbital station with countless research projects ongoing & likely tens of thousand of people supporting the project directly and indirectly by their jobs ?

ISS is not really designed to operate unmanned for any length of time & you could easily loose it due to some simple failure that the crew on board can easily address.

We can't just leave the people up there to fend for themselves. The US needs a capability to get it's people back that doesn't rely on the Russians, and protect it's national interests on the station.