It has the potential to be huge. You never know what's just under the surface until you look. The NatGeo special, "Ten Years On Mars"[1] made a point to include how when a bad tire on Spirit dug a trench in the soft crust, an unexpected surprise revealed itself.
I've often wondered why they don't send a dragline digger to Mars, so they can dig a decent trench. When Spirit's wheel broke and they started dragging it along, digging a little trench, they made a nice discovery and that was with something not designed to dig at all.
If they could get a few feet down, who knows what they could find?!
The Exomars project will land a 2 metre vertical drill on the surface, sampling much deeper than anything done so far.
It's just a question of priorities. Drilling 4cm into a rock with a specific surface abrasion tool is a long way from plunging a 2 metre metal shaft into unknown terrain. Engineering this so it has no chance of jamming, breaking, overheating etc is not gonna be a trivial task. How do you lubricate a drill without contaminating the environment and with such a wide temperature variation?
Makes it harder to what compunds are indigenous and what you just brought yourself. No use shouting Eureka over some organic compound in a hole you just spilled oil in.
In general great effort is made to ensure that probes don't contain any biological contaminates so if we do discover signs of life we're very certain it wasn't a microbe that hitched a ride.
As far as contamination from drilling (due to lubricants or something else entirely) I imagine that the desire for general cleanliness is for much the same reason. If we're going to spend hundreds of millions of billions of dollars on a probe to another world we want to be really confident anything interesting we find is actually from that world and not Earth.
It may have been big, but curiosity has a scoop to scoop up dirt, a drill, and a laser that can vaporize rock to look inside of them. It's not like the rovers can only look at the surface.
