Using a Phone for GPS is an idea I really struggle with.
First, I've seen plenty of cases where smartphones can't maintain satellite fix but my GPS can. Maybe this is limited to slightly older units I have tried. Maybe it's because the GPS antenna in the phone isn't nearly as good as the one built into my GPS. Either way, I don't find a smartphone to be a reliable GPS device, and GPS devices themselves aren't considered 100% reliable.
Second, my GPS is IPX7 waterproof rated, and it is in a ruggedized case with rubber corners to absorb shock. My phone would do poorly if I were to drop in into a mud puddle, creek, or onto a rock.
Third, cell batteries are much more expensive and difficult to find, and don't last as long as my GPS batteries. In a pinch, just about any corner gas station has AA batteries.
Finally, if my phone is my GPS, and while using it I break it or the batteries die, I've not only lost my supplemental navigation device, I've also lost my ability to call for help.
Maybe for casual hikes, it is okay, but otherwise, I am very reluctant to adopt the "multi-use" philosophy in this particular area.