One school of diving that I am aware of is anti-pony. Its adherents are well-represented on this board and they believe firmly in what they were taught. They will not allow for the possibility that in any circumstances a pony is a good idea. They also do not believe in solo diving. One of your diving guru's may have been a practitioner.
On the other hand, at least PADI and NAUI teach that there is a place for a redundant air supply (i.e., a pony) in diving, especially with deep dives. The choice is really up to you.
Personally, I like the idea of a redundant air supply but at this stage in my diving I am not into doubles. I therefore opt for a pony on some dives.
Dives on which I believe that a pony is reasonable are: deep, cold, solo, or any combination thereof. I may well have missed something.
My opinion may be valid, or you may think it BS. The best advice that I can offer is that,
if you choose to dive with a pony,
practice deploying it and stowing it regularly. Since a pony has a small capacity it will run down quickly. I top mine off from my main tanks with a device such as this:
Tank to Tank Equalizer With Gauge reviews and discounts, IST
Safe diving.