• Collections do not equal user groupings
• Attribute values do not equal user groupings
• Attribute values are like blastocysts, they have the potential to become a user grouping
• Organizing your content items (via attribute values) does not equal organizing your groupings

The sidebar is about organizing your groupings so you can navigate to your groupings easily. It's an organization affordance for the organization affordance: grouping.

That's why faceted browsers don't work. Faceted browsers are all about organizing your data. NOT about organizing your organizations of data Holy COW.

The collections that are created under the hood to display things in the summary pane don't always correspond to a grouping the in the sidebar or an attribute value query.