I want to say something really small here, but Dugtrio actually seems like a very good teammate with M-Pidgeot. If you U-Turn on the predicted Heatran switch in, you can pop a potential Air Balloon and simply send in Dugtrio, trap it, and kill it. It also deals well with Tyranitar with Focus Sash Reversal if you really want to go that route. It would give Dugtrio a nice nitche at least.
So, HP Ground isn't particularly needed if you have the right teammate.
So, HP Ground isn't particularly needed if you have the right teammate.