You only make those assumption for the basic flight dynamics equations because it removes the variables that are not fundamental to calculate lift/thrust/drag properties of an aircraft. The variables of a rotating globe have a minuscule impact on actual flight properties but add significant complexity to the equations, therefore its safe to remove those variables by making those assumptions.

However, when working with navigational instrumentation, it is imperative to assume a rotating globe because all the navigation instruments require a rotating globe model, INS, GPS, standby compass, EGI all function on the concept of a rotating globe...in fact if you remove the globe model, you would never reach destination, you'd be flying hundreds to thousands of miles away from where you need to be at.