What are we trying to find or learn that life from earth will permanently remove the chance of?
Species prove to be invasive all the time on earth when exposed to new ecosystems or environments, and usually that's at the expense of many potential discoveries about the life forms that are replaced.
Seeding the universe with life from our planet may make sense as a survival strategy, but there's no ceiling to the knowledge we can glean from life thats different from our own.
In short: we don't know what we don't know, and the gains from such knowledge could be endless.
Why should we? Microbes are not sentient.
> the gains from such knowledge could be endless.
Could be - but the gains from spreading Terran life around will be endless.
Besides, Mars is a lifeless hulk.
From there, https://journals.asm.org/doi/10.1128/aem.33.2.379-384.1977 is "Microbiological profiles of the Viking spacecraft"
Note things like:
> Approximately 1,300 colonies were picked from culture plates, identified, lypholipized, and stored for future reference. About 75% of all isolates were microorganisms considered indigenous to humans; the remaining isolates were associated with soil and dust in the environment.
So that if we do encounter that particular strain on Mars, we'll know its from Earth.
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One that's on the interesting side and has more than just an available abstract: Europa Clipper planetary protection probabilistic risk assessment summary https://www.sciencedirect.com/science/article/abs/pii/S00320...