As discussed in http://forums.musicbrainz.org/viewtopic.php?pid=26102#p26102:

People would like a tool being able to do the whole workflow from ripping the disc, over submitting IDs from disc to tagging the files.

In this mode we start having an entity for a (physical) disc in a drive and connect that to a MB release. We don't show the files as intermediate entities to be moved around.
Therefore a different mode (like in a different tab) would be better than cluttering the "files -> tagged files" mode.

This could potentially done in a different program. Even if not having a way to decide in which mode to start picard is essential.
However, we can re-use the tagging configuration and some other Picard code so and people might like switching mode in a tool better than having to use a completely different tool.