My objectively correct opinion on NMM:
1. It is a good advanced technique to learn. It forces you to understand how light works on metallic (or even just shiny) surfaces because if you don't - the end result looks off. This is a valuable skill to learn as a painter. It also requires smooth blends, which are, again, good to practice.
2. Done well it is an effective way to show off your painting skills. Perfect for competition pieces and display models.
3. Learning it is completely optional. It has its uses as mentioned above, but if you're not interested in trying the effect - there are other ways to improve the same skill areas (like focusing on volumetrics with cloth/skin).
Picrelated is the first NMM study I tried to paint. The gold was largely a failure because I made the highlights too large.