The problem is, if you were to re-use the outputs and then un-Archive the track with the EI at some point, how should SONAR resolve the conflict?
Also, as the page that Noel linked points out, its doesn't make much sense to leave External Inserts active in a project indefinitely because that hardware is likely to get reconfigured at some point with settings that are no longer applicable to the project.
All of these issue are best avoided by bouncing the external processing as soon as possible in the project's life, and removing the EI.