People on the beta newsgroup were wondering why the Media Discovery modules (Add Music, Add Pictures, Add Videos) require that you stop the now playing experience.

So I asked around. Originally (i.e. MCE 2005 timeframe) the framework used
by the First Run modules did not support having a background ‘now playing’
experience. The rationale to not build this support into the framework from the start was because for many First Run modules (i.e. TV Setup) you can’t have something in the background because the module is changing the underlying settings. Then when the Media
Discovery modules were built the team decided to not bother extending the
framework to add this support becuase of the cost of doing so. And so MCE
2005 shipped with you having to stop the now playing experience if you
wanted to launch Media Discovery modules.

However, for Vista we did add this support to the framework. Now users
listening to music can now add music/pictures/video without it interrupting
their music experience. While we could do the same for video and TV there is
not only the additional dev cost but also an additional cost to our design
and production team who need to re-design the First Run template to support
a ‘PIP’ (picture in picture) down in the lower left corner. Now considering
how many other improvements we made to Media Discovery that were truly
important to make (most were post 2 beta so you haven’t seen the changes yet) I think it’s acceptable that we dropped this cosmetic work item to the bottom of the list. And for the most part I don’t believe that people are adding new folders to their libraries every day. Rather this is an infrequent, occasional task.