To me, soundstage should be seen, not heard. If you are in a concert, you can see the leng lui playing an instrument on the front left and some nerdy guy with long hair on the far right. If you pay attention and look at them (provided you have a nicer seat close upfront), you can watch the energy of the person and performer antics playing their hearts out. That's soundstage to me.
However, if you close your eyes, there isn't much of a soundstage to perceive. You just know the chamber, symphony or just a small band is in front of you and you are enveloped in music goodness - where you can feel the vibe, energy, ambience etc. of the performance.
In the car, there's a big windscreen right in front of you and if you look way past that, it could be a road, cars, trees, wall/building so that does not translate to anything close to a soundstage to me. Reproducing the sound goodness would be my goal. We want to achieve this, as long as the music is not running around the car in a surround sound or diffused manner, making it difficult to enjoy.
If you're into watching musicals or concerts via bluray etc. in your home or MPV, it would be a different story. Then we could agree that a sense of soundstage might be of importance. Having a huge-ass HD screen/monitor/tv along with your sound system would help you recreate that sense of visual soundstage along with strong sound presence.
The next time you hit a jazz bar, or hall performance, try closing your eyes. The listening experience will be different.
