Surely, we can all agree that Mio Honda is shit, right?
Scamco says she's awesome, who are we to disagree?
OP's right though. Mio is a selfish little shit.
Forgot to mention that she leaves her unit before Rin even announces that she's officially going to join Triad.
While Rin was having conflicting emotions over the issue, Mio had absolutely no problem leaving without telling anybody until after she left for her own acting stuff.
I know a lot of people will say her and Rin didn't leave, just decided to pursue other avenues of interest alongside NG, but you have to be realistic about these kinds of things.
All things that will eat up their time.
Sure homework and their school lives can be flexible but all the other shit can't. There would be no way in hell that they'd be able to consistently be part of NG anymore.