Right. It’s a work-in-progress that will be exposed in the CLI soon rather than hidden behind an environment variable. Unlike cleanbuild the workflow will be much closer to a native build where you don’t have to re-create containers all the time and running snapcraft commands becomes very fast. See the original proposal.
In this context that’s not true. Once you’re re-using an existing container your parts list may be outdated, and packages installed in the container can be out of date, both of which can prevent you from building your snap.