I have an iPhone and I like to use it in the backcountry - not for communications, but for other apps. It's not a terribly practical tool (due to battery life issues) but I do enjoy it for reading (Kindle app), maps (Gaia GPS), taking photos, and measuring slope angles for avalanche risk assessment (Clinometer app), and collecting avalanche pit data (PitPod).
It's not an essential device, of course. There are lighter and perhaps more ... traditional ways to achieve the same tasks.
What makes it a dangerous device, however, is its communications ability combined with the fact that cellular signals are reaching ever deeper into the backcountry.
Back in September, I spent my 20th anniversary with Stephanie at 9,000 feet in the Wasatch Range of Utah. I tweeted about what we were doing, and I really enjoyed the encouraging replies that came (see photo). Then, I turned off my phone and enjoyed the rest of the day, alone with my wife as we snaked our way up ridges to a high peak at the top.
When hiking with a group, or a pal, or your mate -- the risk goes up. It's bad enough to get the telltale bleep when a text message arrives on a solo day hike, but when others around you hear the bleep, there might be fallout.
Cell phones have some ability to damage, interrupt, and weaken face to face relationships. Just look at the family at the restaurant (you've all seen them) with the faces looking down into their phones while they nonchalantly sip their sodas and chew their pizza.
The bottom line: be aware, be intentional, be careful.
Recently I reflected on the conundrum of having a cell phone with me in the backcountry. Zero bars can be a powerful thing.