I guess there in lies my question - d3 makes it easy to marry the data to a dom, making it easy to add interactivity. So, i guess why would I chose to use Dc rather than to use d3 with crossfilter.js?
If you don't need interactive charts that you can drill-down into, don't use it. If you do, this is the best of breed right now for this kind of thing. dc.js doesn't abstract away the d3 api -- you can still use it to your hearts content, but it sure makes these kinds of examples really easy to spin up with minimal code.
Your second paragraph appears to be the answer to their question, however. dc.js looks like it provides higher level constructs, so you're joining data to a bar chart, not data to a series of rectangles that you need to then make into a bar chart. That's great for people that don't have the time or inclination to learn d3, and since it sounds like you can still get access to the full d3 API, it sounds like it could be useful to people that do know d3, as it would give them good starting points for a bunch of types of graphs that they could then tweak at will.
Of note Dc generates d3 charts...so you can manipulate them to your hearts content. In my case, part of the tabbing functionality (context switch) is to destroy any in memory crossfilters AND hunt through the DOM for any d3 svg elements and delete them to aid browser performance.