Why are per-capita emissions a more important number than net emissions? With the former, China is behind us (roughly half) whereas with the latter, they're roughly double based on the current Wikipedia numbers.
But as for why they are important, I think that there is some relationship between how people live and per-capita emissions. So if you ignore the population size and only look at net emissions, the people in larger countries would be held to a higher standard than the people in the smaller countries.