I think we've seen that the MER *HARDWARE* works well, but that doesn't mean we've extended the envelope of terrain/environmental conditions it can handle.

The system would still be limited to equatorial sites at certain times of the year, due solar cells (and probably thermal issues -- it wasn't designed for polar operations).

The landing ellipse won't get any smaller, and given the relatively small roving range that makes many interesting targets tough.

Would still have the altitude limits of the system.

It probably can't handle high rock abundances, either.

I realize that MSL won't solve all this either. But if it works -- IF -- the smaller landing uncertainty and the long-term power open up a lot of options.

That being said, there were plenty of other sites that were MER candidates. If we're building them cheap we can be more relaxed about things like wind, ellipses with some bad spots in them, etc.

And it is certainly true that five MER clones in less than ideal landing sites beats one crashed MSL....