The simplest answer to the question, Why use NMM when you can use metallic paint? is because you want to. And the answer to the opposite question, Why use metallics when you can use NMM? is the same.
Einion
And that right there is the best answer to every single why and why not question there is out there.
I personally like metallics in a NMM style: the constrast of NMM but metallic shineyness of metallics.
The simplest answer to the question, Why use NMM when you can use metallic paint? is because you want to. And the answer to the opposite question, Why use metallics when you can use NMM? is the same.
Einion