>>6001384imo, the biggest reason is because the rahkshi head only has 1 place where you can attack something. That point is round and only half a unit thick. Because it is round it means that whatever you want to add to it can move around very easily. The metru chest piece prevents this because it fits over the head and down the back of the head preventing this sideways movement.
Thats why people use that piece.
My second best part would be a droid body. Which doesnt move around and can be customized alot.
>pic related 