You might think that just being the first probes to land on Mars to see
what is there and what the planet is like would be enough of a mission
goal, but the big thing about the two Viking landers was for them to
find life.

In the early 1970s, when the Vikings were being built, a fair number
of scientists assumed that while Mars may not have anything more
sophisticated than microbes (there were a few exceptions, of course,
but the days of lichen growing along Martian canals were gone), those
microbes would be right at or just under the surface soil, where Viking
could scoop some up, dump the sample in its little $60 million biology
lab, and voila - Martian microbes!

When it turned out that things weren't nearly so simple, and the Viking
team should have spent more time and resources to better know what
the surface itself was made of, people began to lose interest in the
mission despite all its other successes. The lack of definite evidence
for life was one more blow for exploring Mars, right along with the
Mariner 4 images of just craters in 1965 that made scientists think the
planet was like the Moon.

We should look for life on Mars, but I have the feeling it is not right
at the surface and will require more sophisticated techniques than any
one current probe can handle. And I agree this should not take away
from exploring Mars in its own right, or any other world. Venus is a
prime example of a world that is utterly fascinating and needs much
more exploration, but in part because most scientists don't think it has
any life, the planet has not been given its proper due.