how do you know it's not on a mission? aren't its missions heavily classified? I have no idea what it's supposed to do, but e.g. spying on or interfering with enemy spy satellites would give it a reason to stay up there for a long time.
A device capable of making orbital change already in orbit is a shedload faster to redeploy to some useful orbit, than having to prep for launch.
But against that, they should have 50 of these, if they are really as useful as implied: if not, why isn't the one in orbit suspiciously failing in service? I really don't beleive there is asymmetry in capability to mess with orbital devices here.
A vehicle whose chief feature is that it can take off and land and be launched again not landing and being launched again tells us all we need to know.
If it is just changing orbit while up there, it doesn't need reusability. Just launch another one.
It's is far cheaper to temporarily bring down and upgrade a spy satellite than to build a new one. Probably leads to faster iteration on new designs as well.
Also, the fact that the secret satellite stuff is in the payload bay with doors that can close is likely very desirable when a Russian or Chinese nosey satellite matches orbit. And once that does happen, the ability to move to another orbit is on tap as well.
I'd say that the fact China has launched their own nearly identical copy is evidence that the X37 is not useless, but China copying someone's space thing is just another day ending in y and not actual evidence of anything except their copying prowess.
Copying is how China and Japan became industrial powerhouses. There is no shame in copying, except where you have been tricked into copying something useless.
In political jargon, a self-licking ice cream cone is a self-perpetuating system that has no purpose other than to sustain itself. The phrase appeared to have been first used in 1991–1992, in a book about Gulf War weapons systems by Norman Friedman, and On Self-Licking Ice Cream Cones, a paper by Pete Worden about NASA's bureaucracy, to describe the relationship between the Space Shuttle and Space Station.
And now we have SLS, and a planned lunar space station whose only purpose is to be a place SLS can get to; the moon itself being out of reach.
Make no mistake, a lunar space station, no matter its name, is not a gateway to anywhere. The only defensible place to pause on the way from low earth orbit to anywhere else in the solar system is high earth orbit. Stopping at high lunar orbit is pure cost, no benefit.
It might be useful to park spare fuel in low lunar orbit for a moon landing, but there is no value in a space station there.
Apparently they park it in orbit instead of a hangar to make it look as if it is actually "on a mission".