Why do we still have mitochondrial DNA?

The mitochondrion isn't the bacterium it was in its prime, say two billion years ago. Since getting consumed by our common single-celled ancestor the "energy powerhouse" organelle has lost most of its 2,000+ genes, likely to the nucleus. There are still a handful left—depending on the organism—but the question is why. One explanation, say a mathematician and biologist who analyzed gene loss in mitochondria over evolutionary time, is that mitochondrial DNA is too important to encode i