I'm not going to venture any guesses as to their motivations or reasonings, but I remember them talking at the
Blogger's Day thing about the focus groups they did when they started designing the PRS-500. If memory serves me (it was almost a year ago, and I didn't take notes on this part, so ... ), they took a group of folks and had them sit down and play with some Libriés and started with what they thought of those.
I'm not real familiar with the feature set on the Libriés, but the differences between them might be informative, in that they presumably changed them based, at least partially, on the responses from the focus groups.