If every living species were either a host or a parasite or both, then there would be no doubt that parasitism was an integral property of life.
Unfortunately, every living species has not been studied to the depth necessary to ascertain this information. However, enough species have been, so it does seem reasonable to make this assumption.
Why bother? Because the opportunity to see if this property is restricted to Earth, or is universal. Mars is currently being investigated. All properties of life should be looked for. If parasitism is found on Mars, then we can be more confident that parasitism is a universal property of life.
If life without parasitism is found on Mars, then we will be able to see why life on Earth carries such a parasitic burden.