Short-lived staging environments (eg, a temporary clone of a production environment) certainly have a place, such as when making architectural changes, but these kinds of changes are generally not happening on a regular basis if you are making incremental changes and doing continuous deployment.
I'm sure that there are companies that have a valid engineering need for perpetual staging environments rather than feature flags, but I've seen absolutely no evidence that staging servers are commonly engineering driven. Certainly in every part of the web startup world I've had contact with or heard about, staging environments have consistently been for product QA in organizations with heavy-handed processes and/or product management by non-technical stakeholders.
Edit for the people responding: This is not about haphazardly pushing to production. You should familiarize yourselves with continuous integration (http://en.wikipedia.org/wiki/Continuous_integration) and the various deployment strategies of major web companies.