My thinking is that investing in training is a one-time deal, and if you practice what you learned, it's a skill for life. That's good value. There's no additional equipment involved to ensure you and your buddy don't run out of gas. As I see it, I learned in Fundies to do more or less what we were all taught to do in our basic OW course but perhaps didn't learn it properly: use the buddy system.
So while I agree with your thoughts here, in practice it isn't so. In my earlier post I described from actual experience, only about 20% of divers on Con Ed, can complete an OOA exercise to a satisfactory standard as teh donor, despite being briefed and having a dry run on land.
The OW course is, overwhelming for a new diver - there is a lot to take in, OOA is one of many skills, and of course there is the basic buoyancy and divign skills to get right. Most people qualify and then "forget" or chose not to practice.
It's a sad fact that many people forget how to put their kit together or carry out buddy checks properly - so more "advanced" skills have no hope.
On a guided dive, this is really not an issue, because your buddy is in the case of an emergency, the group or guide, so I would agree that on this kind of dive a pony is not a necessity, and indeed a PITA.
When you diverge away from guided dives, and become reliant just on one buddy things change. If you know your buddy well and have decided that you can rely on them then yes, don't carry a pony.
If the answer is "maybe" or "no" then carry a redundant supply.
Divers seem to embrace the "new fashion". Pony's, Long hose, top of the range PDC and BP/W often pushed as the way to do things on the internet.
I still see no need for long hose for rec diving, indeed 2 people I've "tested" in a OOA scenario have both failed miserably, I almost died of boredom waiting for them to stop faffing (and one I had to put their alt reg in their mouth).
I'm happy to let them and others embrace their equipment choice, I just wouldn't' rely on them in an emergency, unless they've demonstrated that they can use it correctly before hand.