What if I adapt your code under MIT and distribute it. Can someone else then use it commercially? Or do they pay you? Or me? The answer is your license either isn't open source or it is easy to bypass.
Dual license copy left on the other hand works because the stipulations and restrictions are compatible, i.e. not overlapping.
Is it obvious? court cases have a way of exposing the edge cases of 'plain english', as well as a general tendency to interpret ambiguity against whoever happens to benefit from it. Lawyers would tend to describe what you're doing as a 'crayon license', and it's often an apt description. I would not like to wind up in court over this license, on either side, and I will as a general rule avoid them.
What if I adapt your code under MIT and distribute it. Can someone else then use it commercially? Or do they pay you? Or me? The answer is your license either isn't open source or it is easy to bypass.
Dual license copy left on the other hand works because the stipulations and restrictions are compatible, i.e. not overlapping.