We recommend using ERG mode as little as possible.

The reason being, there is no ERG mode when you're riding outside (and particularly in a race!) Smart, quick gear switching is a skill athletes need to have on the bike, and you don't get to build that skill using ERG. Those micro-adjustments of the small changes in cadence when you switch from one gear to another take a physical toll on the body, in a good way. If you spend your training time in ERG, not manually making adjustments, you're not getting the training for what you'll go through on race day.  

Obviously using ERG from time to time is fine but you're short-changing yourself if you use it all the time.

Bottom line: The best cyclists in the world do both.