Let's say I'm a competitor, and I find that Niel (randomly picked) is someone I want to hire. All else being equal, I offer him $100k (website says he's making $88k). He comes to his boss to say "I like it here, can you match it?" What does his boss do? Especially, if he's valuable to the company... What if I have a very specialized skill that doesn't fit nicely into your matrix? Let's say market pay for my skill is…
It's my job making sure everybody is paid fairly. If Niel came to me and told me about a competitive offer, I'd say, "well, let's look at the market; maybe we've fallen behind." And then we'd sit down and refresh the data that drove our current pay levels. If things had changed significantly, then it would be time for raises. If not, then we'd talk about why a company might be paying well above market, and whether it's the right choice for Niel to go there.
For specialized skills, yeah, a new matrix category's the obvious fix. If co-workers are the kind of people that freak out at market economics, then it means we're not educating them well, so that's something worth fixing.
I think it's important to remember that secret salaries rarely stay secret, so the choice isn't between full knowledge and zero knowledge; it's between full knowledge and half knowledge plus substantial speculation.