As for the walking feature, the "reworked" plot revolved around human-Pokemon relationship, and the protagonist being the pure hearted trainer Ho-Oh shows up for. The walking was justified in-plot, and can easily be justified in-plot for Black and White considering what the protagonist is fighting for.
Actually... that sounds like the main reason to why they should have had walking pokemon in BW... I mean didn't most of the story have to deal with all the good guys trying to explain to team plasma as to why People and Pokemon should be together?!
I still don't know about this having so much extra space on the system (And I assume the DL'able content takes up less than 5MB space due to how they set it up), the way it was designed seemed to be in a last min. cut mentality. They knew people wanted them, and seemingly added it to the dream world to just save space (With things like the houses, berry growing, and taking pokemon for a 'walk'). Maybe Iwata was pressuring for a fall release, or maybe the sum of the parts exceeds the allotted left over space. (or maybe it's a combination of both) I just don't feel it was gamefreak's fault, and looking at how much the addressed fan desires with this game makes me not think they were lazy.
I just really hope we don't see gen 6 for a long time.