Just a couple things to keep in mind when using a smart phone as your gps:
1) Waterproofness?
Unlike most dedicated gps units (such as Garmin, etc.), most smart phones are not even sweat proof ---- just a little moisture can mess them up. Yes, you "can" work around this with the proper after-market case ---- just note that most dedicated gps units are in fact waterproof right out of the box.
2) Battery life?
Using your smart phone as a gps can drain your battery quickly.
If you're using the same phone for both communications & as a gps, you can find yourself with neither when you may need both.
Personally, I typically carry a dedicated gps, a smart phone I also use as both a gps & for communications, and a separate "emergency" phone (carried in a small crushproof, waterproof case). Plus, extra power backup (which is simply extra AA batteries for the dedicated gps). I do like using the smart phone as a gps (love the bigger screen), but there are times the dedicated gps is more practical, such as navigating during a storm, such as needing that phone for talking, while navigating.
Carrying an extra old smart phone to be used as a gps sounds like a great idea.
Nice to have so many choices today
Doesn't seem that long ago to me the only hand-held gps units had tiny monochrome screens with no mapping software, and they were relatively expensive considering today's capabilities.