It's the phase difference between two or more signals that is of
interest.  It is used by the ear to do all kinds of localization
(someone on the list will be able to explain that a little bit better)
and as a result can be used to give an impression of height.  A constant
difference of 90 degrees gives you a cosine for every frequency
component (i.e., sine) in your signal.  As was pointed out to me by Sean
Costello, this has been used for analog frequency (pitch) shifters.

Job van Zuijlen

Francisco Vila Doncel wrote:
[snip]
> . So, why worry about phase?
> 
> I'd like to know if i'm right.