I did consider some factors, including the ones you mentioned, when drafting my original message. I just didn't include my considerations into my message. I prefer addressing factors on-demand rather than attempting to immediately address every possible factor, especially for topics such as this one where the number of possible factors is virtually unlimited.
My point is that, by default ("generally"), there is no need to run multiple instances. But I admit that sometimes it can be justified, such as if you (truly) need zero-downtime deployment or N+1 redundancy (which is questionable, because, as senko pointed out, services usually go down for reasons that aren't a power outage or a fire). My complaint wasn't really about this anyway, sorry for not being clear. I mentioned that my company runs multiple services "for absolutely no fucking reason," but I did not state their intention, which is not zero-downtime deployment or redundancy, but rather "scalability," and I find this pointless because a single $5 VPS could easily handle many times the amount of traffic their IO-bound app receives. The end result is that they now must always think about the implications of running multiple instances, and failure to do that properly creates race conditions and other kinds of obscure bugs.
On the other hand, if you only run a single instance, reasoning about the system becomes much easier, and even though that does not eliminate the possibility of a race condition, it becomes much harder to create one. I also like the architectural benefits of a single instance system, such as being able to keep transient state in memory worry-free.