I've carried people in the game before and it's not that far fetched from a concept. I've gotten to understand why people do carries.

Some just can't invest the time into learning primals, some are too scared to walk up to parties and try it out, some don't want to deal with people, some don't want to deal with RNG rolls, some people just want the story without dealing with a lot of complicated mechanics.

The fact of the matter is, you do learn something from carries. Hell, I'd even go as far as to say that you learn some extremely fast recovery methods just via osmosis.

The person I carried a dozen times through Shiva Extreme ended up learning the majority of the fight and actually contributing to the carry's progress up to a certain point. That's when a carry is just solely to get a single item and not worry about getting counter rolled.

I carried a few people through T5 and T9 as well, both of them were single carries just so they could either get the story or get to Final Coil.

There is no harm in doing carries, the concept is essentially you're hiring seven mercenaries to help you defeat an opponent. In concept it's much the same way as crafters crafting items and selling them, and gatherers gathering items and selling them. You could bring up the argument "just learn to craft or gather and do it on your own" but you won't.

A person's earned money is theirs to determine what fits best to spend it, if your friend decided to buy a dozen carries rather than the 4 milion minion off the market board people put up then it's their choice. You've got to respect that otherwise you'll end up landsliding into the elitist category.

In regards to your friend offering 5 million gil to carry them through Bismarck Extreme, why didn't you offer to set up a learning party for him instead? You clearly didn't want to carry him so you just told him a hard "no, go learn the fight" which in the most likelihood just meant he ended up paying more for someone else to carry him.