Nick Bostrom often talks about this. Once you hear his thoughts on why its bad, its hard to disagree with him. I went from being excited about the prospect of finding life on Mars to hoping that we don't.
Nick seems like a smart guy, but I disagree with him and his reasoning. Here's a quick summary of his argument, and why I disagree with him:
His logic here is that there's a great filter - where "great filter" exists. The "great filter" defined as a difficulty in getting from step A to step B in the climb from primitive life to a galaxy-wide civilization. For example, we could define these steps as (0) no life, (1) primitive single-celled life, (2) multicellular life/animals, (3) intelligent life, and (4) galaxy-colonizing life. It's possible that the great-filter is a giant weapon of mass-destruction which kills off life between steps 3 and 4. It's also possible that the great-filter is simply a result of the difficulty of getting from step 0->1, or 1->2, 2->3, or that no space-faring technology exists. Humanity is currently at step 3.
The Fermi paradox suggests that there's a problem getting between one of these steps. If we find life on Mars, it hints (but does not prove) that getting from step 0->1 is easy. This suggests that there's an increased likelihood that the great filter exists between 3->4 (we might all die at the hands of an advanced earth-killing alien device or that space-faring is impossible). He thinks that getting from 1->3 should be relatively easy. I should note that I don't think his belief is substantiated by anything. Personally, I think getting from 1->2 probably isn't hard, but it's easy for me to believe that getting from 2->3 is difficult (either because most planets don't remain life-sustaining long enough evolve intelligent life, or because evolution tends not to produce intelligent life with much frequency). Basically, there's a bunch of variables at play and to suggest that simple life on Mars increases the chances of killer alien technology seems to be based on a thin thread of mathematical speculation. It's sort of like saying, "If your girlfriend is late getting to your house, then it would be very bad if you found out it was raining outside, because if it's raining, then it increases the probability that she was in a terrible car accident. Therefore, the combination of 'late girlfriend' and 'rain' are very bad." The logic is sound, but that doesn't mean you should suddenly start thinking the worst because it's raining outside.
Hypothetically, if Mars had fully intelligent life on it, then it would suggest that getting from 0->3 is relatively easy (therefore, we should see massive numbers of alien civilizations), and that 3->4 is difficult (perhaps some early alien race is killing-off all the others, which means we should be afraid). But since we have no real mathematics to calculate the probability of getting from 1->3, we can't really reach much of a conclusion about killer aliens coming and destroying us between 3->4.
tldr: Don't worry about it. Primitive life on Mars doesn't mean much of anything.