Although the "groove" around the rear glass does look nice, it seems to be the place where any dirt that's rinsed off the window tends to accumulate when it rains. Then when the car drys, the groove is filled with dust or dirt. The same thing happens to the top of the rear bumper cover because it's completely flat and not slightly tilted down at the back so water runs off. Sometime "styling" takes precedent over practicality.
Both issues are probably only noticed by us "A-type" personalities who wash their own cars.
People who take their X3s to automatic car washes never get to learn these details.