I disabled it mainly for one reason: the ill-conceived placement of the Siri button on the keyboard. I use the keyboard all the time, but I only use Siri from time to time.

When I am trying to type, I shouldn't have to worry about where my left thumb is hitting when I need the ".?123" button.

As far I know, there's no way to keep Siri enabled but to remove that button from the keyboard.