Getting rid of the backend - frontend dichotomy sure is an attractive idea but I don’t think it’s the way to go. Don’t get mistaken, frontend editing can be improved and it most probably will. For instance, having the tree handy from the frontend would be great... Also, the inherited admin and web users dichotomy should disappear.
I don’t know about security concerns choosing to get rid of that dichotomy and giving access to backend features in the frontend, but I can say that from a usability standpoint, it suck IMHO. Drupal is a good example : the admin is cluttered, though Drupal 5.x has greatly improved things by grouping items and a more "sugar coated" web2.0-like design, I find it poorly usable... Overall, the interface is still the same linear/hierarchical logic and lacks visual hierarchy and intuitiveness.
The way I see things, interface have to be "layered", i.e primary / most used information first then second, third layer get you get more detailed or advanced feature. Also contextualizing things is key to user experience. Context menus and help are a great addition. This is all quite obvious to say, much less to achieve
Some have based their success getting the ligtweight, 20% features admin that gets 80% of the job done (think 37signals and Apple here). I think it’s a good target for the future of MODx interface, and the process is already in the works.