in either case, what do you do? if you can't reach a box and it's otherwise safe to do so, you just reboot it. so is it just a matter of which situation occurs more often?

The thing is you can survive memory exhaustion if the oom killer can do its job, which it can't many times when there's swap. I guess the topmost response to this thread talks about an earlyoom tool that might alleivate this, but I've never used it, and I don't find swap helpful anyway so there's no need for me to go down this route.