Why should I bother with Frequency Response calculations in a Spilt-rail buck regulator design?
It's all a matter of proper control loop design in order to meet performance objectives.  

If the compensation is completely wrong the output of the converter may oscillate.  

If the phase margin is too low you will get lots of ringing in response to transients.

If the loop bandwidth is too low, the converter will be unable to respond to fast transients and the output will sag under load transients.

If the bandwidth is higher than necessary for the application the output may be noisier than necessary.

In addition it's important that the compensation be robust, so that variations in component values due to tolerance (inductors +/- 20%, caps +/-20%, ESR maybe -50% +300% over lifetime, etc.) don't cause instability. If small changes in loop gain can cause large changes in control response (e.g low gain margin) then the converter may not be stable in production.