In an emergency you might want to make a phone call, and searching for the SIM at such time might not be preferred
Redundancy is one thing, my Nexus 5 with SD is also on hand if need be, but the best redundancy is a good old paper map in the bag in case the whole GPS network is down (solar flares, terrorist attack, space debris, whatever). A third GPS device won't do you any good when that happens.