Well, with this I just wanted to point out how little sense Rainbow Dash's statement from the Season 9 Opener actually makes.
I'd also like to address here in the description the times the princesses actually didn't help:
-In the Season 4 Opener (the plunder vines), the Season 6 Finale (Chrysalis' second attempt) and the MLP-Movie (the petrifying orbs) they were incapacitated, by exceptional means.
-In the cases of the Flim Flam Brothers, Starlight Glimmer, the Pony of Shadows and many minor events, the princesses weren't even asked for help.
-The Season 3 Opener (Sombra) was explicitly a test.
-At the Gala in Season 5 (Treehugger-episode) Celestia wanted such chaos to happen. Of course she wouldn't want to stop the 'fun' too early. The notion that she couldn't use her magic because of a tiny blob on her horn is so ridiculous, that she must have faked it (again, to not stop the fun). Seriously, what would stop her from lifting her hoof and wipe the blob away?! And if Discord would have thrown Treehugger into another dimension… well, then Celestia would have brought her back or would've forced Discord to do it.
And on top of all that, there's something in that scene (where Rainbow makes the "almost never help"-statement) that many seem to have missed…
At the end of the scene RD says to Twilight:
"We've almost always done things without the princesses' help, which makes sense now, because it was all just training for you to take over!"
Seems like that is now the official way of addressing this 'princess-help-issue'. You can call it an oversimplified explanation, but it was clumsy writing (including Rainbow's first statement) that made that explanation necessary in the first place.
So to all people who say "The princesses are useless!" and that this 'uselessness' is officially acknowledged now:
Yeah… not really.
All images belong to Hasbro.