I think whether something should be a core feature or a plugin depends much on how specific it is. Something very specific for one field, e.g. draw roofs or pipes, or as mentioned above introducing a level concept, doesn't fit into the main program. Something much more general that can be used in all fields like live solid tools fits much better, even if not every user would use it.
Edit: The level feature I mention was from the other thread by the same name, not this one.