In general, people carry two regulators for two reasons: One is for self-rescue, should there be a problem with one or should it be lost, and the other is to share with another diver who, for whatever reason, has no breathing gas. Some people elect to put one regulator on a tank on their back, and a second on a completely separate tank (pony bottle). Others put two second stages on the same first stage, coming off the tank on their back. A very few people carry no additional second stage at all.
Once you have decided to carry an additional second stage (and possibly first), then you have to decide which of the two you will donate to someone else in the event of an emergency. Standard teaching is to donate the "octopus" regulator, which is kept on a slightly longer hose, and fixed somewhere on the diver's torso. People who use a combined octopus-inflator (Air2) donate the regulator in their mouth, and switch to their backup. Those of us who dive a longer hose/bungied backup configuration also donate the primary and switch to our own backup. Someone diving with a pony bottle may donate the pony regulator, and perhaps even hand off the tank, depending on how it's configured.
I would say that, unless you only dive solo, it's rather selfish not to have a second regulator to share with someone in need. But I have to admit that my secondary regulator has mostly come into use to save ME -- a bungied backup is extremely handy, should you fall on entry or exit and be unable to put your hand immediately on your primary regulator.
In my opinion, your friend's advice is bad.