I don't see why we shouldn't change Ubercart's dependence on other modules during the 5 to 6 port. After all, Drupal itself is a dependency.

Porting extra code to Drupal 6 for Ubercart's use means that we take on the responsibility to maintain it. Taking on this responsibility for TAPIr, uBrowser, and Workflow-ng is not something we want to do. Doing away with the limitations they impose on the code will make the work done on the 2.0 branch easier in the long run.