Since the demise of Symbian OS, the ability for native use of external GPS receivers has vanished from the operating systems (not sure about iOS), so it cannot be used by the applications, and in some cases it is very needed. These cases include bad weather or terrain conditions where external, more sensitive GPS unit would perform far better than the one built in in the most (and especially cheaper devices). However with both Android and Windows Phone, such units are practically unusable. Android has a form of hack-ish solution by enabling mock gps data in the developer menu and using a software that would feed the real data through the mock data ability, but still this is not an every-user solution.
Because external GPS receivers are not supported on OS level each app needs its own support of this functionality and so far I saw none of the major apps to support it. Everyone relies on the location API of the OS. In my view adding such support would not be that complicated and most of the external receivers communicate in standardized way by using NMEA standard for the data transmitted, which is quite plain, well documented and does not require any complex processing.
Not to mention the impact on battery life of the device when you have your OS location services disabled and still be able to use navigation software.
Not to mention the impact on battery life of the device when you have your OS location services disabled and still be able to use navigation software.