Whether it's an iPhone or not, there are advantages to bringing a device you're carrying and using every day. I chose phone and watch for every day use, but with backpacking, running, etc in mind. I wanted to be able to use my gadgets for as many of my activities as possible even if they aren't optimal, in favor of not having to worry about having 10 different gadgets that may/not talk and all have different UI. Sorta like setting up your tent in the backyard every day and hanging out in it while you do other stuff.

I'd rather not struggle with a camera if I were to see something really cool (cute marmot, breathtaking vista, Bigfoot...) and stumble off the trail and break an ankle. I can find other ways and occasions to break an ankle, thankyouverymuch.

The glaring disadvantage of course is if you lose or damage your phone, you're either stuck or you have to rely on a backup that you're not as familiar with.