Humanity has not demonstrated the ability to effectively sanitize spacecraft. The risk of intentionally bringing life to Mars is that we tromp all over evidence of extraterrestrial love.
The absolute most valuable science to be done on Mars is to determine how unique Earth is.
It would be a bummer if the sterilization techniques used on the unmanned landers hadn't worked, but if they didn't, then life is already multi-planetary.
Also I really don't get this fixation to "not contaminating Mars" - like, why ? Its one of the few usable planets in the Solar System! Of course we will contaminate it all over with life, possibly turn it terrestrial one way or another.
Anyone advocating against using one of the most important next stepping stones for Humanity must really live in a weird bubble.
Sure, pick some Kuiper belt object or some extrasolar planet and declare it a reserve and we will make sure we won't touch it when we get there. But doing the same with Mars is insane and almost seems like some weird irrational religion.
The absolute most valuable science to be done on Mars is to determine how unique Earth is.