Wow, blast from the past. Yeah, this issue really did piss me off. The whole idea of having a portable, is that you can play the thing on the go, and sometimes you're not going to be near a wi-fi hotspot or anything like that. Our family goes camping quite a bit, and we would always bring our handhelds to get some gaming in here and there, and not being able to play head to head without an internet connection really sucked.
I never did try Fifa 13, I wonder if they added ad-hoc for that version ?