The MERs have a limited set of environments where they can land -- has to be equatorial (for the sun), near "sea level" in altitude (for the parachutes), and not too windy, or they just wont work at all. Probably some constraints due to reentry trajectories, too. So ice caps and tall volcanoes are simply out.

That still leaves plenty of ground to cover, though. And we could take risks with unsuitable terrain -- being too rocky wouldn't guarrantee failure, but we'd have to live with a coin toss about whether it survives or impales itself on a boulder. On the other hand, there are plenty of areas so rough that they'd be no point to even trying, since even if it survived landing, it would never be able to get anywhere.

One question I have is how much it'd cost to build clones of the rovers. LEss than the first two, I'm sure, but at this point it's been several years since they were built and we may already be losing some of the potential advantages of re-using tooling, supplies, and expertise.