Well it sort of is. If someone tries to argue that say communism isn't that bad because Stalin killed less people then Hitler, but the actual between them is 20 mil to 25 mil there isn't that huge of a moral difference. Especially because such numbers are rough ballpark estimates who's margins of error overlap a lot.
There are some interesting things you can learn from looking at the number though. ie if you classify mass murders in terms of orders of magnitude (tens of millions, millions, hundreds of thousands, etc) and add up their scores, then it turns out that a small handful of regimes at the tens of millions level (Mao, Hitler, Stalin, and maybe Imperial Japan, King Leopold and Chiang Kai Shek) are between them responsible for most of the violent deaths that happened in the 20th century.