The mitochondrial genome is still present and used in most (if not all, I do not know) species that contains mitochondria. The importance of the loss of the mitochondrial genome is not the same in all branches of the tree of life.
I hope this answer to your question.
An interesting factoid: the transfer of mitochondrial genes in the genomes has led to the reclassification of some species that were thought to have diverged before the endosymbiotic event to a very different place. I can’t find the reference but it was a paper written by (inter allia) Hervé Phillipe.

there are different hypotheses why mitochondria haven’t lost all their genes so far. one says that some very hydrophobic proteins cannot be imported into the mitochondria from the cytoplasm (which has also been experimentally proven) and interestingly the two most hydrophobic mitochondrial proteins have never been transferred to the host nucleus. Another hypothesis refers to the modified genetic code these organelles use to translate their mRNA into proteins. It says that some genes would just not build the right proteins upon expression from the host nucleus (e. g. TGA is a stop codon usually, but encodes tryptophan in human mitochondria).

That was actually quite enlightening, I had never even asked the question why the host synthesizes many but not all of the mitochondrial proteins. Those examples had nice logic in them.