Jeff made some good points about issues with smartphones, that some have already addressed, but to toss in my 2 cents ...
"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."
I agree that this is a valid concern. For me, what matters ultimately is the empirical rather than the theoretical; I've used the GPS built into multiple smartphones in the past and have had overall good results. It's not just the antenna, it's also the chipset built into the device, and these have been getting better. I think it's also a matter of expectations; I realize that a multi-function device typically doesn't do "best in class" at any of the functions.
And now adding GLONASS helps IMO to mitigate time to lock on and ability to lock on and accuracy. This web page gives a good sense, I think, of what GLONASS adds to the picture (and I would add that in "the real world" those times when I pick up more satellites my smartphone does so much better at locking in and giving me an accurate position fix): http://www.oxts.com/default.asp?pageRef=134
"Either way, I don't find a smartphone to be a reliable GPS device, and GPS devices themselves aren't considered 100% reliable."
I don't know how to respond to this one; a heart pacemaker isn't 100% reliable, but it's hopefully "reliable enough". I've never had an issue with a GPS or smartphone failing to work as designed, and I've used a number of alternatives over a number of years. I don't mean to debate the value of learning to use map and compass (!), but I don't see this as an argument to factor in.
I will agree that some GPS units are designed to be more tough and/or waterproof, and in fact most backcountry GPS units are indeed more so than most smartphones. It's another trade-off. In something like 9000 miles of backpacking I've always carried a GPS-enabled smartphone in a neoprene or similar case on a pack strap and have never had an issue as a result. Well, up to last week, which I think was somewhat of a special case, a couple of pieces of bad luck had to line up in that situation (lost my smartphone in deep snow while leading a snowshoe trip). And I could have lost a GPS in just the same way.
"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."
Agreed, per above, but ... also question how significant a factor this is, per above. My smartphone in a snack-sized ziplock does just fine against rain. Sometimes a bit of a PITA using the touchscreen that way, but in the infrequent times when I need it, no problem.
"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."
This is actually the prime reason that I carried a standalone GPS on the CDT --- one encounters a lot more situations where a GPS is a helpful on that trail, and being able to carry spare lightweight lithium batteries and use the GPS as much as I wanted without thereby compromising my ability to take photos or to journal, that made it worth carrying a separate GPS. But for most trips, including long distance trips, I find that a spare battery or two for my smartphone is plenty. Really, cell phone batteries can last quite a long time with some education on how to limit default-mode power drains.
"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."
I think that you're saying that by keeping the smartphone handy (to use as a GPS), you expose it to more risk, whereas if you just had a dumb phone you might wrap it in something protective and store it deep in your pack --- something along that line?
Acknowledged. Since my phone is an all-in-one device for me, I always keep it handy anyway, so for me at least there's no incremental risk exposure. And I've got a pretty good track record at carrying the phone with no problem. Leading a snowshoe trip last week was a real exception for me; I think in part it had to do with the distractions of "playing leader". On solo trips I've just not had a problem with pulling the phone out to take pictures or infrequently make a voice recording or whatever while on the trail.
A related point is that carrying a separate phone, GPS, and camera (and maybe more devices) adds to weight carried, as well as complexity. And I really like that my photos end up on my internet-capable device, ready to be uploaded directly. And that if I end up hiking somewhere unanticipated, I can download maps on the fly for my GPS. For example, that happened in 2011 when early season creek crossings were so fierce my hiking companion and I had to bail out of the Bob Marshall wilderness and do one long-assed walk-around. Unanticipated walk-arounds are typical on the PCT and CDT, more commonly due to fires. But there are a lot of potential situations where a person can be taking a long walk and end up going somewhere they hadn't planned on from home.
"Maybe for casual hikes, it is okay, but otherwise, I am very reluctant to adopt the "multi-use" philosophy in this particular area."
Each to their own, of course, but I hope I've been clear in laying out the logic "on the other side" of this, for why the smartphone multi-use approach can be a good one for more than casual hikes.