One simple reason I have done it is because the requirement was made mandatory in our SOP. I know for a fact, the first carrier where I flew the 400 was instrumental in ensuring the long range nav procedures were carried over from the classic to the 400.
Jammed Stab raises the issue of checking distance, L/L and/or track distances for a crosscheck. I can not tell you how many times I have seen the wrong L/L entered and the distance and track match what the correct route should have been. In addition, when you have a re-route it's not a big deal to plot it out. I might be a simpleton, but I believe one picture is worth a thousand pictures.
Let's be realistic, in this day of CPDLC/ADSB, taking a minute to do a quick plot is not going to upset the work load. It might upset your power nap, but that's about it......