See, the thing is, Using the mouse to aim is much, much better than the keyboard. It not only allows precise control over the artillery barrel, it is much, much smoother then a keyboard when trying to fire at moving targets. If anyone has played The Deluge, a mod similar to NW, the artillery system has cannons which utilize keys to aim. Personally, it feels terrible. The cannon has a fixed distance per key press, meaning that the artilleryman is only able to aim within the preset ranges. Example: say there is a target at a certain range. The cannoneer aims and fires, but it goes over the target's head, so the cannoneer presses the down key, and fires again. Now it is too low. If the cannoneer were to press up on the arrow key, it would only go over the target's head again. Thus, there is absolutely nothing the cannoneer can do to hit the target, simply because of the restrictions that are applied to keyboard aiming. < That is frustrating as hell. The same can be applied for the horizontal axis. Whether the current cannon system is a benefit or a handicap to the artillery system, it allows much more control to the cannoneer, and it feels much better then using keys.
And honestly, if you are saying that the mouse control makes the cannon too inaccurate, that isn't really the cannon's fault. I have absolutely no problems aiming a cannon with mouse movement, and I am able to accurately drop rounds into lines within a second of a cannon being completely loaded because of the mouse-based aiming. Trying to aim with the keys would be slow, choppy, and overall inefficient.
Just think about how you would try to aim with the keys. You would not be able to make many of the aiming possibilities that you currently have. What about aiming diagonally? Would using keys make that better? Heck no. The Artillery system in this game is easy enough, I don't think we should resort to taking all of the entertainment out of it with a silly and inefficient control system.
If you want to try out keyboard-based aiming, download The Deluge mod and use the cannon. You will see what I am talking about.