Outside of the "base training" context, I view it as filler in between intensity sessions. Even at that, its need is probably determined by the athlete's intensity and volume. I've been doing 15+ hour weeks with a couple of high intensity sessions thrown in each week...if I deviated too far outside of zone 2 for too long on the easier sessions, I'd burn out pretty quick...or the quality of my intensity sessions would really degrade. If I was doing <6 hour weeks, I probably wouldn't even bother looking at my power meter, outside of interval rides.