Also they really over do the fact that they are completely different part.
Rarity and Fluttershy are completely different too, but at least they show that they have several things that can keep them together. The spas for one, and Fluttershy having a knowledge of sewing, Rarity needing her to take care of Opal, etc. Fluttershy isn't in my top 3, but there's much better stuff between them than with Applejack. And the "opposite" stuff can still be used there. (Rarity craving attention, Fluttershy not, Fluttershy living relatively simple where Rarity doesn't, there's stuff to work with.)