This is my thought on it, do you realy want your phone to be everything, what happens if you get a call while your looking at memoery map?, also dont for get the power drain on the bit of kit, the more it does the more it uses, and more chance of batery failing at the wrong time, also how many items can your phone have with bluetooth connections at the same time so if you got your phone bluetoothed with a hands free kit, then can you blue tooth a GPS with it?
I like the idea of a seperate system dedicated to mapping as you have the info there, I have been in the position of being on the phone trying to locate someone by phone.
I also carry a set of OS maps and a compass with me incase the system fails.