They are used more than any other monitor, but they are not usually the main monitors. They are a standard, but not a reference. Someone somewhere, sometime said wow, these are good for mixing and they catch on. Soon enough everyone had them as a standard. It's like Lexicon is a standard for reverb, or sm-57 or 58 is for guitar mike and vocals.
There are probably 100 better mikes but they are a standard because they are studio and touring proven.
Mastering is not the same as recording tracks and put them into a mix. But you probably know this anyway.
I interned for 2.5 years in a large studio in Philadelphia while I worked on my undergrad. Every studio had NS-10 or some flavor of it. But ALL had main monitors where the majority of the work is done. NS-10 give you another perspective. The studio I interned for directly used 3 monitors. KRK's, Genelecs, and NS-10's for near-field montoring. Studio 1 had some gigantic set of mons and NS-10's and they had an old time radio w/ one 4" speaker that ran a mono mix for a completely different perspective. All connected to a Neve board that cost 1.5 million dollars at the time, because there were only 3 like it in the country.
It's all about perspective. What's the final mix going to sound like. But to lay down good tracks and to finally put it all together, you need a really good reference.
Scott wrote, "help me find a high end speaker kit suitable for mastering audio" If he would have said, "I need a recommendation on a set of mix speakers? What should I look at? I would have said, KRK, Genelec, Event, JBL, And Yamaha. But that's not what he wrote.
Just for fun, take a look at
http://www.wilsonaudio.com
I don't disagree w/ NS-10's. I would get a set too if I were setting up my studio.