It is not a matter of just parking an extra capsule. Even dormant, they have a window of time for remaining in space.

A true, dedicated vessel for safe abandon and return to earth (lifeboat) has never been developed. We just always rely on the vessels used to get them up there.

On the ISS, NASA is in charge of providing electricity and thermal control. Roscosmos is in charge of providing maneuvering thrust and excitement.

“when complete, are supposed to be able to support multiple crews at once, so they shouldn’t have to worry about unoccupied stations, barring some really, really bad luck.”

Like thrusters firing out of control?
https://behindtheblack.com/behind-the-black/points-of-information/thrusters-on-soyuz-docked-to-iss-fire-improperly/

It does not even have to be attached to a docking port. Its remote controlled anyways.

I wonder if a fake docking port could be attached to the structure just to store one? Data and power hook up of course.

To expand a little on what Bob said:

The new seat swap agreement is for assurance that there will always be at least one person on the ISS to operate the US part of the station (USOS), or conversely, one person to operate the Russian modules (ROS), in the event that a crew needs to return home early. This is because if one person becomes severely ill or badly injured, the whole crew must return to Earth, since NASA policy is to not leave someone up on the ISS without a way to return to Earth. Currently, that would result in the USOS being unoccupied if the Crew Dragon needs to return (or the Russian side unoccupied if the Soyuz must suddenly return). Something NASA would like to avoid if possible.

Hopefully it becomes clearer why this difficulty will still remain for NASA even once Starliner is operational. The USOS is not going to have two crew vehicles simultaneously docked save for the brief intervals of crew handoffs or commercial mission visits (like Axiom’s this spring).

Now, I’m merely describing NASA’s reason. I am not endorsing it.

I think….the longer term answer here may be when Axiom Space starts installing its modules on ISS in 2024. That will add additional docking ports and additional life support, and NASA could probably work up a deal with Axiom to have one of its crew trained at all times to teemporarily take over USOS maintenance if the NASA crew suddenly had to leave. By the way. the proposed Axiom, Northrop Grumman, and Orbital Reef stations, when complete, are supposed to be able to support multiple crews at once, so they shouldn’t have to worry about unoccupied stations, barring some really, really bad luck. At that point, NASA could achieve its objective even if it had to completely break off from the Russians.

Col Beausabre: The barter deal really centers on safety. The manned capsules are the station’s lifeboats. It is essential that everyone know how to fly both.