We invented this feature in response to bloggers and site owners who wanted to be able to test a new plugin before adding it to the live/production version of their site. We're also the only managed WP platform to offer such a feature as standard with all accounts.
I can see that in a true dev/staging/production test suite environment, it does't stand up as a true mirror of product for the reasons you stated - but then neither does the staging/local version of Google App Engine or Heroku offer the same exact environment of their production platform.
If you need true like-for-like mirroring of production and staging, then yes, you need to create another WordPress instance - that's the case for most providers. Also its wroth nothing all WP Engine accounts, other than the Personal plan, come with multiple WordPress instances, so rarely that means actually buying anything further.
You may be the only managed Wordpress platform to do so, but App Engine has had this functionality for a long time.