Common yeast can survive Martian conditions

We should be dropping bags of extremophile organisms into the Martian atmosphere to get a start on terraforming it.

Then we will never know if mars once had life that's different from earth life

We still find evidence of life on earth from billions of years ago that was not erased by billions of years of their successors. I doubt you have anything to worry about.

But imagine the value of finding _any_ living RNA based life which is clearly not from earth before we travel to other stars.

It's one thing to know there was once life, and to know basically nothing about it. But being able to date it in the tree of life (or forest of life?) is monumentally more relevant to understand our place among the stars.