I agree, I love Luna and Celestia but the writers always insist on putting them on a bus every time theres a crisis.

Couldn't agree more, save for the bit with holding back the storm. The Princesses were still active, instead of just sitting around or going offscreen while Twilight and literally only Twilight tried to fix everything. And seeing as they needed a magical artifact of "God knows where this shit even comes from" to hold back the winter previously, them doing it still shows they have power.

As for the rest, yeah…. It's completely nonsensical. For the reason, I really think it's just incompetence with the writing as they had no idea how to involve so many characters with the plot. Which is why 80% of the time it's only ever Twilight doing anything. (And, well, I don't think it's a coincidence that all those two-parters have the same person as the writer. cough cough)

And it's not exactly like the Princesses are the only issues with those episodes… But still, I hope this is at least them understanding their mistakes. And they figure out how to get them more involved when it's necessary.