I loved it! It gets an 'A' from me!
Rainbow Dash didn't abandon Pinkie, but she did get swept up in Cheese Sanswhich's hype, which can happen. When the time came where Pinkie thought she had to leave, Rainbow and the others were right there to tell her how much they needed her, and that they didn't want her to leave. One can be loyal and still get enamored by the new and exciting. It's what happens when the chips are down, that's when loyalty is tested.