I think one government, or rather something like eu, but for whole planet, will be useful, and it will be possible when live standards for people in different countries becomes the same, and old feuds are forgotten. However that's not something to happen in near future and i do hope that mars will be colonized and become off of the planet entity long before that:)

The problem with one powerful government per country is that different countries still have different powers and they like to meddle with weak ones, like US and Russia did in this case.