When doing sound I most often use the phase reversal on the drum kit mics. Some guys bring in a bunch of mics for their kit, and with that many mics phase can be an issue. I try to keep it simple with a kick, snare, HH, OH, OH, and Tom. But some guys want every little toy mic'ed, so what can you do. It has also comes in very handy with acoustic instruments that have both an internal mic and a pickup. Sometimes they will be out of phase. During sound check I always check phasing on these types of things. Sometimes the difference is night and day. It has also been helpful in a situation where a band is doing a single mic style show, except I like to run xy cards right underneath the LD mic so I can get better coverage. There have been times when doing this that it sounded much better with the LD out of phase to the SD's down below.