Those two sisters are awesome. Most fans think they don't do much, but they're actually useful when you think about it. It would be nice to see an episode where something happened to the mane six and the two sisters had to rescue them and save Equestria.
i actually have my own theory's on this...the powers to control the sun and moon are inherent powers that they can more freely control like they were connected to them, but bringing the moon or sun into the atmosphere would not be a good option for them because it puts everyone in danger. so they have to resort to basic magic which is not enough even with their enhance alicorn magic. it is shown how they tend to always depend on a group effort to solve worldly matters. while they control the celestial to keep the worlds nature in balance so their is not termoil preventing peace and prosperity, but since Twilight's talent is magic itself and her ability to resonate and make that power even more so with her friends around she can handle earthly matter more effectively. hence why she becomes the peace ambassador for the kingdom.