The single most important reason is this: An n-body system makes it impossible to easily predict what's going to happen ahead of time. With the two-body system in place now, you can set yourself on an interplanetary trajectory, then observe the SoI shifts hundreds of days in advance. With an n-body system, however, you would have to simulate the entire path ahead of time, instead of simply doing a bit of math to predict where you'll be. Also, it renders timewarp basically impossible but that's another story. As such, it's not going to happen, ever. The advantages of the patched conics system are far too great.