AC analysis on transient model of Buck converter
AC analsyis (in the sense the term is used in SPICE) assumes all components in the circuit are linear. 

A buck converter only works if some components (the switches) operate nonlinearly. 

Therefore, an AC analysis will not capture the useful behavior of a buck converter.

That said, if you consider only frequencies well below the switching frequency, you may be able to use an AC equivalent circuit, with the the switches and inductor modeled as a controlled source, to analyze the control loop and verify such things as the loop stability and phase margin.