Depending on how bad your layer lines are and how thin your walls are, the entire part being an imperfection is very much possible. Also, if you are doing 5 coats of paint, it's easier to get the whole thing pretty smooth if you did a good job sanding without coating the whole thing in putty. A lot of builds I see doing the bondo slurry are doing sanding, bondo, primer, then paint and that's it.
I agree though, most people can get away with just sanding and using the bondo for fixing small areas and hiding welds. The standard build would do well to do a sanding pass (80>120>220) first before the first layer of paint/primer goes on. I see a lot of layer lines through the paint because there was not enough sanding done.
