Sorry, my previous post was missing a few important words to clarify what I was thinking:
Spot is more useful for a trail like the CDT rather then the AT which stays near civilization and has better cell coverage. While you can't be sure of having CELL coverage everyday on the AT, going more then a day without it seems rather unlikely. If you don't mind paying for the SPOT service (ends up costing more then the device cost) and carrying the weight, then sure, carry it. Keep in mind that many GPS can temporarily loose a signal in heavy forested areas, so I'd assume Spot would be the same.