I'm not really talking about cell phone manners. I'm talking about the actual necessity of having a phone with you at all times. RandyT touched on the idea. Say you're in an accident where minutes mean the difference between life and death for yourself. What are the odds you are able to make a phone call in that situation? You're nearly dead, right? Can you still coherently access your phone and use it in that situation? How did people managed to not die in those situations prior cell phones? To me that point of view sounds more like a scare tactic than a reasonable possibility.
Another one I hear a lot, mostly from women, is "I have kids in school so I need to be reachable at all times in case of emergency." I have two kids in school and there has never been an emergency where a parent was urgently needed. Yeah, they have been sick at school. The school may or may not make a call to have them picked up. Yeah, one has been in trouble a couple times. Again, they call, and the kid hangs out until I show up. It should also be noted my kids' schools don't call cells first. They record which number is a cell and try landlined daytime phone numbers first. When I asked about this I was told "we are often told after leaving cell messages that the parent is not allowed to use their cell at work and thus did not get the message." So, at least the schools here, prefer landline contact numbers.
It is a valid point that there are very few payphones anymore. Odds are good, though, that if you are in an area where a payphone would have been you're also in an area where a landline can be used. A couple of times I've had a car break down, walked into a small business, and been allowed to use their phone for 5 minutes to call AAA. That's how it was before cells, right? How often is anyone in a place where they can't find someone who would take $5 to use their cell phone to call for roadside assistance?