Great little feature, as a pilot you don't have to constantly calculate the 3 degree direct path anymore (in case you get shortcuts/vectors). It just shows it on the ND.

Lets be honest, in the real world, the FMC/FMS calculates a perfect descent profile giving best efficiency. Unfortunately, ATC will not often have you descend VIA. Most likely they will step you down in chunks based on their needs, airspace and traffic. As the pilot, you have to be proactive and anticipate your descent progress. Some FMCs/FMSs have tools to help you calculate outside of the mental math. Some, such as this feature, makes it easier to calculate and visualize where you stand in regard to your descent. This allows you to make adjustments early on, such as slowing down or asking for earlier descents. You can also tell ATC right away that you can't make depicted or ATC given crossing restrictions.