Seems like a statistics problem. Chances of finding life vs. chance of mission failure. If the chance for mission failure is too high, you won't find life anyways.
I do agree with your sentiment that they should try to minimize the chance of failure before ruling it out, but I don't think it's "bad" to consider this chance.
(Now for stupid example that oversimplifies things to make it seem like I'm correct) If you have one mission with a 10% chance of finding life with a 70% chance of success, that's a 7% chance of finding life overall (despite if it's there or not). If it's 15% of finding but 40% success, that's only 6% chance of finding life overall.
Now if only we knew the actual percentages...