It seems to me you're saying that this is the simpler way of doing it, but you haven't really responded to the proposal that this somewhat perverse measurement system leads to perverse outcomes. To take a somewhat absurd hypothetical, let's say Vatican City started producing natural gas very, very efficiently. They would be increasing their emissions but very clearly decreasing global emissions if they were able to displace coal, etc. But given that their present emissions are presumably virtually nothing....
Doesn't this measurement problem have consequences for us in that it discourages us from developing resources that might actually improve the environmental situation globally?