Well, at least one user needs this feature (me).  Why wouldn't you include it?
The main reason I want this feature is that I don't want to restart the server each time my assets change.  (The server keeps data in the process which I don't want to save to db and restore very often.)
Here's how it is now, before adding the option: 
  - server is started, preprocessors are run in `add`
  - we change the asset sources, morbo sees the change and restarts, so preprocessors are run again in `add`.
  - we refresh the page and see our changes applied

Here's how it will be after adding the option:
  - server is started, preprocessors are run in `add`
  - we change the asset sources, server doesn't restart
  - we refresh the page, the preprocessors are run in `expand`, and we see our changes.

An improvement, in my view (: