Actually Earth ponies possess strength far greater than that of Pegasi or Unicorns. Maud demonstrates this when competing with Rainbow dash in rock throwing. And alicorns are a combination of all three types, otherwise that battle with Tirek might not have been survivable. I mean. You need to be physically strong to withstand being thrown into a mountain by a centaur and still have the energy to fight. Physical strength and earthly connections make them important to their civilizations. A majority of farmers in MLP are earth ponies.