Well, Sombra got one point right, Celestia sure do not left a good impression of a good princess, first she gamble with her student life to get her sister back not even assisting Twilight in case Nighmare Moon decide " screw useless traps, I do no longer want to scare them, I will just go to them and kill, it will take less then minute ", where was she when mane 6 searched for elements giving Discord many chances to attack, when Changelings invade she failed in fair combat because she did not even bothered to use her brain in battle, and now she gamble entire empire well being, her student and her friends lives, live of Captain of her guards and live of princess.
After season 3 finale I know that it was all to see do Twilight is worth to become Alicorn, but the way she teach and risk she take just show her as a bad mentor in my opinion, I just hope that in season 4 she will finally DO something, same for Luna, no more silly excuses to being useless.
She runs a kingdom, she has to put well-being of it and majority of ponies and whoever else live there above well-being of Twilight and her friends, who actually serve as an elite military forces. Also as a ruler Celestia probably is a little too busy with politics and keeping economy in check. I suppose so does Luna.
Celestia may be a ruler, but she is also Powerful Alicorn, if powerful being waste her powers by doing paper work, they become sort, and in time of need they do not know how to use they huge power to protect they subjects ( it would be one think if Celestia would be Unicorn, as Unicorn she would not be able to help much, but as Alicorn it is her duty to protect her subjects ).
Also, you are not busy to rule Kingdom in time of crisis, when crisis come, all small unimportant decisions can be put aside and full focus come to saving the country.
Also, Twilight and her friends are not elite military force, they are group of civilians who were manipulated into becoming elements of harmony, and as elements of harmony make them not expendable, if one of them die, elements become useless, so it is in Celestia best interest to protect bearers and assist them.
It is like group of terrorists would set up bombs and kill those who try to defuse them, would you send Sapper without any escort to defuse bomb when he would be targeted, or if there is only one doctor who can save someone's important live and there would be group of people who would want to kill him to prevent it ( of-course you ensure that Doctor safely ), mane 6 not one pony army, they can KO super villains only if they have elements and only if they are close to target, and most of the time mane 6 not even have elements with them, giving enemies tons of opportunities to strike them down, mane 6 are glass canon what can not be replaced, they hit hard but also can go down easy, with Alicorn escorting them at last they would be steel canon.
Lucky Twilight is now Alicorn so hopefully mane 6 will become steel canon, and thanks to this villains will stop doing stupid mistakes to lose.
However, it also could be that they are protecting country or even the whole planet from some enemy we haven't got to see and which is much scarier than everything we have seen before. That would explain why are they busy all the time. Though, also will make things creepy. I mean, where all these villains come from?
Hmm...Well, maybe the whole point of mane 6 fighting creepy and power hungry creatures was to see if Twilight is worthy of being an alicorn? This would also explain why exactly none of divine sisters interfere. Thus Divine Sisters seems to be "sink or swim" mentor type (ok, trickster type in the best case). But then again, alicorns are serious business and you can't turn somebody who can't deal with life or death situations quickly.