I do admit that Celestia has taken a lot of risks with Twilight and her friends. In the "Crystal Empire" Luna clearly wanted to get involved herself. Sure she liked Twlight but you can't blame the princess of the night for thinking Twilght isn't advanced enough to take on a dark force such as Sombra. I mean Twilight could handle the whole friendship stuff while Luna handled the combat if it came down to Sombra getting through the barrier.
I do find myself wondering what it's like to be a guard in a world like that. Slooooooowly losing musculinity in a world ruled by mysterious emotion-based forces............................aw gawd, I'd probably sprout a tiara a few weeks in Comic artists, make it so!
When Discord was there,she and Luna had to protect Canterlot,and well,the rest of Equestria. If Canterlot fell to Discord,that mean a lot of trouble. For King Sombra,Princess Cadence AND her Royal Captain of the Guards(if he's still that) + the elements of harmony were there.
Discord: They needed elements of harmony to defeat Discord. How would they protect Canterlot from him? And if Canterlot fell it would have still meant nothing to Equestria. Because Discord had the elements and was already terrorizing local population.
King Sombra: Both of whom were all but useless as the Royal captain of the guard was beaten as if he was a colt and the princes of love was turned into giant battery for the shield. And if Celestia had not tought her student the black magic then so would the elements of harmony. Oh wait. There were no elements of harmony in there, just a couple of representing ponies with no abilities to defend themselves. Way to go Tia. Considering it took both her and Luna to defeat him last time, I think that pinching in by perhaps sending Luna to at least occupy the good king would have been the least she could do.
"A relatively normal day is disrupted when Luna learns that Orcus, the Demon Prince of Undeath, has escaped from Tartarus and is making his bid for conquest, starting with Ponyville. More horrifying though... is that fact that only she seems to care. Time is of the essence, and no one, Celestia included, seems worried about the impending disaster. Will Luna be able to rally the army, shake the citizens of Canterlot from their strange apathy, and save the day?