Mitsuki is a casualty of plot necessity. Her love being unrequited prompted Haruka/Yuu to evaluate the kind of relationship they have and want to have. Or at least it should, I didn't read the manga past anime events.
Meh, dunno, I felt the conclusion of that love triangle was just as bad as the love triangle itself...
I mean, they kept kissing, getting jealous, even considered marriage at some point, but they don't even get what "love" is ? Oo That left me totally dumbstruck how stupid that was...
I enjjoyed sakura trick a great deal, but trying to force some love triangle in it wasn't really a great idea in my opinion. Those works when you don't know for sure who will win, but in that case, it was obvious from ep1 that those two were head over heels for each other and Mitsuki had no chance at all...