Grant Wahl, a soccer columnist at sportsillustrated.cnn.com, uses a similar measure: the baseline is "win at home, tie on the road." If you can do that, you are guaranteed to advance.
So, for Mexico at Azteca so far this cycle, they are -6 (three ties at home means they gave up two points each game). They are +2 on the road (one win, two ties) for a net of -4.
The USA by that measure is also on top of the group at +1 (two home wins = 0, one road tie = 0, one road loss = -1, one road win = +2).
The nice things about this measure is that it helps equalize for different number of games played, and different number of home vs away games.