High precision GPS will be really helpful for SLAM, though, because when you know where you are, you have much fewer features to sift through (pun intended).
I agree generally, especially if you are exterior but it fails completely with overhang or poor overhead coverage. Better to have a system that can do everything without the third reference as it will be less cumbersome.
The other thing is that a CDGPS doesn't initiate with high precision like a camera does so you will always have (arguably) a higher amount of correction you need to apply manually if you initiate from that reference as opposed to with visual odometry.
