Man, looks like I need to clarify the entire purpose of having the sideslip indicator. Using it to fly straight and level is only the simplest case.
Any time your plane has Yaw AoA (that is, its nose isn't aligned with the direction the plane is traveling), you get sideslip and the ball moves away from center. This happens while rolling, while in flat turns, etc. The ball moves in virtually every maneuver so long as you don't use rudder to correct for it. Contrary to what the masses think, the rudder's main purpose is to correct for sideslip - NOT to set the proper deflection for a HO
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The main point of keeping the ball centered with regards to ACM is to reduce drag by making your airplane fly true into the wind - this allows you to retain more energy.
As for realism - ask any real pilot. If they fly with sideslip, they can FEEL it. They don't need a ball to fly approximately without sideslip because it just feels weird when the ball isn't centered. A good analogy is when you are turning in a car - you don't need to see the road to know that you are turning. You could be blind, but you can still feel the fact that you are turning. In fact, the inertial Gs you feel while turning in a car are basically similar forces to sideslip. Flying without sideslip basically means you no longer feel the side-to-side forces.
The reason it isn't gamey is precisely the fact that WWII pilots could directly feel their aircraft sideslipping. The lack of such a mechanism is what Hitech calls "false realism" - sure the pilots wouldn't have something taped to their eyes to tell them the sideslip, but they could feel it all the same.