A quick calculation is required because your computer ain't gonna tell ya. Does your buddy have enough air left for both of you to get to the surface safely?
What do you do if your buddy's remaining air can't get you both to the surface safely?
What is your buddy going to do when they realise they don't have enough air for you both?
There is a concept called "rock bottom", or "minimum gas", that will avoid this issue. You can find information about this in many places on ScubaBoard. Having you and your buddy not know whether there's enough gas to get home is not an indictment of the buddy system. It is a sad statement about what you have not been taught.
Every diver has to face the question, "What do I do if I have nothing to breathe?" Granted, in the absence of inattention, gas failures are rare. But they do sporadically occur, either through freeflows or blocked dip tubes (there are not many things that will almost immediately render you without any breathing gas supply!) Each of us should think about what we are going to do in that circumstance, not because it is likely, but because it is potentially deadly. (In the ER, we say, "Think about the most common thing, and the most dangerous thing this could be.")
There are various ways to address the issue. One is the presence of an attentive, trained buddy, who maintains minimum gas reserves. The system in which I dive mandates that, and the principles work. It's possible for any buddy team to learn enough to reserve adequate gas, and to practice to be able to execute a gas-sharing ascent calmly and smoothly, but a lot of people don't practice. And if you travel alone, and get on boats by yourself, I think you have to be prepared to deal with such an event on your own if need be. I do not think you can be certain that the person with whom you have been randomly buddied will either have enough gas for you (unless you thoroughly discuss this ahead of time, AND the person respects the limits) or will be able to provide gas and remain with you through a smooth and controlled ascent.
If you are such a traveling person, I can easily see wanting to provide yourself with your own redundancy. I mean, I do it when I dive with new or out-of-town divers; I choose a benign site, and I dive my doubles. But most people don't dive doubles, and they aren't easily available at a lot of resort locations. Having an auxiliary bottle, whether you bring it yourself or you bring a stage kit and sling an 80, makes you much more independent of the group around you.
I don't travel alone; the only time I have had to dive with an instabuddy was when there was a mixup and I showed up on the wrong day for a charter. My experience of that day was that I was able to remain with one (but not both, since they made little effort to remain together) of my buddies, but it was at the cost of seeing almost nothing for myself during the dive. I would not do that again. I would rather dive with an auxiliary bottle, and dive alone, or as part of a buddy pair or trio where I really didn't care that much if I lost the others.
I don't dive with a pony. I don't dive with random buddies, if I can avoid it. But for people who have to, I empathize. Better to carry your own redundancy, than to have none at all.