Is adding a GPS cradle to the iPod touch worthwhile?
No info to report on the cradle you linked to, but several Bluetooth models work fairly well. Apple apps work the best but some third party mapping apps don't use the GPS as well as the onboard one for the iPhone. Others resist installing as the developers require iPhone or iPad with GPS to install. 

It would be best to contact the developer of your mapping app of choice and see what they recommend. 

http://www.gns-gmbh.com/ - tested that for a day so it worked but I can't recommend it for the field without more time with it.

XGPS 150 - this worked well but I ultimately sold it since I don't want a short days GPS but a whole weekend to a week so I'm back using my garmin that is all in one and takes AA batteries.