I'd think that a dedicated GPSR would use less battery than a Smartphone? I'd really need to see some statistics showing phone battery life with the various radios (GPSR, Bluetooth, GPRS) switched on and off to get some idea.
Personally, I like the idea of keeping my phone safe and secure and not running the batteries down - just in case I actually need to use it in an emergency. Having another device that does route logging etc doesn't therefore bother me and, like I said above, for multi-day rides, everything needs to be working off replaceable batteries (or you're going to need to rig up some solar/dynamo charging system).
The point isn't to track where you've been, but to report where you are, and when you're not moving, after decreasing acceleration rapidly.
I reckon I could knock up an app to do that, as all the functions needed are exposed by Google's API.
Seems silly to me to have a device which is tightly coupled to another device, which provides the ability to replicates it's capabilities. And doesn't cost £130.
Any of the reasons to not use a mobile phone for this specific use, also apply to using a separate device. Although as an additional con for a separately paired Bluetooth device, you need to have another radio switched on, and BT is pretty power hungry.
As another option, I'd provide an encapsulated device - I think it's one or the other, personally, and a more elegant solution.
NINJA EDIT :)