Rocael,
Basically the same thing Caroline said :)
I think originally the plan was to merge them back together. The code came out of client projects. The main reason XCMS is seperate is that I did not want to break anything in BCMS-UI-BASE that people were already using.
Last time I checked, XCMS uses the cr templating features more completely than BCMS-UI-Base. Besides that I simplified the handling of the administration forms. XCMS also uses the site-wide categories package.
XCMS also (last time I looked) integrates better with the default subsite templates. It does not define it's own form or list templates. This makes BCMS-UI-Base more flexible in presenting the UI, and make XCMS fit in with the rest of the Admin UI.
Both pakcages use the content repository in a similar way, so switching is a possibility.
THe key is that no user interface for content management is going to meet all your needs. Customization will always be required. Overall goals are to make the customization easier by defining clear points to change it. This can be accomplished by making it easier to define content types, generate forms automatically, and possibly define workflows for authoring and publishing. .
