When you draw the panel lines in with black on top of the final paint coat the result is cartoonish and very unrealistic. In the real world panel lines are NOT 100% black. You only get absolute black if there's no atmosphere to look through... like on the moon. On Earth, shadows (and panel lines) are never pure black.
I read about this method on one of zoom zoom's posts, and judging by what I've seen of his work, the result is very realistic. I'm paraphrasing here, but basically what he does is paint the body, then draw the panel lines in with black ink... and then he sprays a couple of color coats over the top (I'm talking airbrush here), but with the paint cut 50/50 with clear. The result is that the final diluted color coats cover the black panel lines just enough to knock back the stark blackness, but still leave them a realistic shade or two darker than the overall color.
Simple and effective. No muss, no fuss, no fooling with scribers, sanding, etc., and great results. That's the kind of technique I like!