The fact that NPS (and measures like it) are predictors is useful, in that you end up also collecting real growth data, and the difference between NPS-predicted growth and actual growth can tell you how much of your growth was organic vs. marketing-driven, and thus give you a picture of your brand’s “inertia”—how valuable it is just sitting there doing nothing, separate from any marketing projects. Tracking that allows you to judge the brand impact of everything you do. But you can’t do it without some measure that actually gives you brand value, rather than simple growth.