I have found often when typing on mobile device I use @, underscore, and .com incredibly often. Factoring in social media and @’s usage increases as does the #. Currently to access these keys I need to leave the keyboard I am on, access another one, and search the layout for where these keys are placed. Then once the key is selected I must go back to the original ABC keyboard I was on.
In original layouts these keys were given a lower heirarchy, most likely extending from the desktop days when Qwerty was king.
But as our needs change, so does design.