GPS is convenient, but it has other advantages as well. It's almost always available, it's highly accurate, has a quick time-to-fix, etc.
Inertial navigation systems require finicky setup and drift over time. (Ask the passengers of KAL 007 about that one.) However, the setup can easily be automated using GPS inputs, and the drift can also be corrected that way. This way, you have two systems which help keep tabs on each other. The INS can't be spoofed, and so can tell you when your GPS goes out of whack. The GPS won't drift, and so can keep your INS up to date when it's not being spoofed. Cost aside, two systems are better than one.
