I think they want the wind. It cleans off the solar panels. Staying close to the walls is dangerous from falling earth (well mars) from winds and planetary movements like earthquakes, if there are earthquakes. Sticking to the middle of the crater i think is best bet as it wont dirty the panels as quickly, as well as allows more sunlight. When needing to sample, that would be the only time i would go near the walls, if that was necessary. Im more concerned as there was rumor that Opportunity has a wheel thats goin bad (but is not bad yet), to which they can not do what they did with Spirit's bad wheel to get out of the crater from the slope. This should be an exciting slow mission.
