From the perspective of Evolution there are tradeoffs between sexual and asexual reproduction. Sexual reproduction allows recombination to occur, creating new combinations of beneficial alleles while allowing harmful mutations to be unlinked and purged. Plasmodium has a life cycle where it reproduces asexually in humans, but undergoes sexual reproduction in mosquitos.
Not all single celled organisms reproduce asexually. Many Eukaryotic single celled organisms have a sexual reproductive cycle, even yeast.