Yes, you can get assistance with your iPhone at ANY AT&T or Apple store in the country...though id highly suggest sticking with Apple stores for help in all situations.
As far as the coverage...if you plan to keep your phone for 3 years, then yes, i guess it would be worthwhile...i rarely keep mine over a year (as that is the frequency on new iPhone releases), so to me its not worth it...but long term, $70 sure beats full price or used price even 2-3 years down the road if it fouls up and stops working.
FYI...iPhones are quite reliable, and serious problems usually show up quickly, keeping you within the 1 year manufacturers warranty...