"Are you presupposing this rock varnish on Mars has proven visible lifeforms in it?"

I'm supposing it is similar to rock vanish found on Earth, I don't much about rock varnish. Let's see:

"The orange, gray, brown, or black veneer coatings have physiochemical and biological origins. Rock varnish is made up of a coating of clay particles (approximately 70%) combined with mineral deposits of iron and manganese oxides - - all probably wind-blown material that settled on the surface of rock. Other minerals mixed into varnish composition include hydroxides plus silica and calcium carbonate. These ingredients are cemented to the rock surface by living bacteria. The bacteria reside within and beneath the microscopic layers of varnish, and are usually absent from the exposed surfaces. Exactly how rock varnish is formed is not completely known, but one theory is that varnish formation is a means by which these microbes protect themselves in their exposed, extreme environments. Manganese oxides in rock varnish block the transmission of ultraviolet radiation. Could the rock-dwelling bacteria be simply be creating their own effective sunscreen?"
http://eduscapes.com/nature/rocvarnsh/index1.htm

Maybe earth rock varnish is from Mars. It doesn't seem to be of vital importance.

Oh, here some more details:
http://www.geo.arizona.edu/palynology/geos462/14rockvarnish.html

Some kind of plant life- needs CO2 and sunlight.

Oh, wiki got some:
Desert varnish forms only on physically stable rock surfaces that are no longer subject to frequent precipitation, fracturing or sandblasting. The varnish is primarily composed of particles of clay along with iron and manganese oxides. There is also a host of trace elements and almost always some organic matter. The color of the varnish varies from shades of brown to black.
http://en.wikipedia.org/wiki/Desert_varnish

And:
"Clay, then, acts as a substrate to catch additional substances that chemically react together when the rock reaches high temperatures in the desert sun. Wetting by dew is also important in the process.

Another important characteristic of desert varnish is that it has an unusually high concentration of manganese. Manganese is relatively rare in the earth's crust, making up only 0.12% of its weight. In desert varnish, however, manganese is 50 to 60 times more abundant. This significant enrichment is thought to be caused by biochemical processes (many species of bacteria use manganese)."

Oh, under table:
"Table 8.2 A few of the literature misunderstandings surrounding rock varnish and its environmental relations. See chapter 10 in Dorn (1998) for a more detailed accounting of the history of thought on varnish. "
http://alliance.la.asu.edu/dorn/VarnishPages/GeochemicalSediments/GeochemicalSediments.html

It says:
Misunderstanding:
Mars analog: Assuming that varnish is a biogenic rock coating, the occurrence of Mn-rich varnishes on Mars provides evidence of life or former life.
Resolution:
The use of varnish as a bioindicator of Martian life is untenable until abiotic origins of Mn-enhancement are falsified for Martian conditions.

So, we don't have proof either way, but otherwise may be possibly. And rock varnish exists in Antarctic- coldness doesn't seem to deter it.

Also at latest link is this:
"Four general conceptual models have been proposed to explain varnish formation (Fig. 8.9). This section first presents the four proposed hypotheses of varnish genesis and then evaluates strengths and weaknesses."

I take from this that they don't know much about it yet.

But reason used it as example, is it is possible.

Btw, wiki is wrong when they say: "when the rock reaches high temperatures in the desert sun"

Oh, also, fire and the wheel can't really be called scientific discoveries- though Penicillin can. So, my bad.

As for your question about value of science. In order to have value, science must have some practical use by humans- it must better our lives. Now, it doesn't need to do this in the short term- basic research generally doesn't have short term application. But the value is it might lead to some application- which somehow make living better.
And of course, science has value to a scientist because it's exciting/fun or whatever for that scientist.

Now, let's compare studying black smokers and rock varnish- I'm not doubting that some scientist may have a bunch stuff they haven't published about or published I haven't a clue about. But from my vantage it seems there will a lot more known about either them. Both are important, but I tend to imagine that smokers might be more important