Ah, but sand isn't just sand, and dunes are of different types and sub-types. and have myriad local shapes and textures...

Don't totally knock the surrounding plains. I'm sure that if we pottered around there for 6-12 months, we'd find interesting soil types and even distinct rock outcrops (look what Spirit has wrung out of a boring volcanic succession - to the point of making it actually not-boring!)

But I agree that there comes a point (about now) where there is value in going down into a deep crater. We will certainly learn a lot. Maybe we will acquire a lot of data, and just possibly, we will get out again afterwards.