Think about how fast a point on the equator of the Earth moves relative to a point a few cm away from the North Pole.

In one full rotation of the Earth, the point on the equator will have traveled 40,000 km, and the point by the North Pole will have traveled a meter.

So… it’s that it’s a useless way to express rotation.

The same thing that’s wrong with saying I’m going 1250 rpm down the road. It may be correct, but doesn’t actually mean anything without more data.

Because linear velocity and angular velocity (the speed at which an object spins) are two different quantities, as is already apparent from the units (m/s and rad/s). Saying that something rotates with 1/4 c is simply not a useful statement. It would be useful if you were saying that, for example, an object at the event horizon travels at that speed. But that speed is dependent on your altitude above the massive body. The same angular velocity (rotational speed) at greater heights translates to lower speeds than further below.