I have found them to be extremely useful in waking up the nervous system and invigorating the energy in my body infact when i first started i would get an instant erection it felt so good and when over a strong sense of calming bliss would go over me and energy for the day ahead. I do not know why i did this practice but it sure helps dealing with kundalini.
I do know of a yoga teacher and his partner who claimed to have had cold showers everyday for a long time over a number of years because they did not have hot water (they lived on an island) and they really praised them and said they were wonderful- so maybe youre onto something! I always like taking a bath until the water is very tepid, luke warm, and sometimes even cold depending on the weather. It slows the heart and aids relaxation and is very soothing! Water is so therapeutic!
I have found from doing it everday several time a day that over time the intial shock dies down to the point it is not hardly felt i feel it making my nervous system better able to cope with very livly energy in my body.
Have restarted this practice about a month ago. It sure helps with energy disbalance, improves immunity and aids detachment from the senses. Its also supposed to be a powerful practice for developing the svadistana chakra.