They may choose to play high and force the offense to be far away from the goal but then a through ball and a fast striker burns you.

The back line is able to dictate the movement of the other teams offenders so it's a bit of a chess match between offense and defense.

I do agree that if any part of your body is onside then you're onside. I hate that if just your pinky toe is offside then you're all offside.