I doubt that the level of life succeeding enough to damage our ability to determine if there was already life there is on the same order as "terraforming a planet". I doubt there would be much upside.
Sure - by 'terraforming' I just meant 'replacing Martian life with Earth life'; terraforming from the microorganisms perspective perhaps, not from the perspective of humans. There's a lot of work involved in that, way beyond our current engineering capacity.
I'll make my thinking more explicit ...
The best case is that we either a) don't introduce microbial life, or b) it just flat-out dies. Then we can examine Mars for signs of life (current or historical).
The worst case is that we contaminate Mars with microbes that flourish there, forever destroying our ability to examine Mars for signs of life.
I'm assuming we can't explore Mars without risking the worst case. So it seems worthwhile to try; the very worst case is that we replace a Martian microbial ecosystem with one from Earth.
