Why use cross-spectral density to calculate frequency response?
in the audio biz, we call this the "Two-channel FFT".  the cool thing about it is that you can measure the magnitude response of a room or something using music (that is decently broadbanded) as the test signal (and divide the output spectrum by the input spectrum).  you don't need to pollute peoples' ears with an impulse train or a maximum-length-sequence or pink noise or with a swept-frequency chirp.