In the aftermath of the Canterlot Invasion by the Changlings, Twilight assured her friends she forgave them for not trusting her. Rainbow Dash know better.
You only hurt the ones you love, but that works both ways. Things said in anger, things done in anger can linger for a lifetime. And there’s no guarantee you can ever take it back.
What's one of the more embarrassing things you could ever have happen to you? Getting a wingboner in front of all your friends seems pretty high on the list. Twilight is of course grateful for her wings, but they do have their downsides.