What exactly is would be the disadvantage of running something in stating environment before running it "for real" in production? I'm assuming the staging envir

What exactly is would be the disadvantage of running something in stating environment before running it "for real" in production? I'm assuming the staging environment is an exact clone of production (except reduced size: fewer app servers + smaller DB instance)? I understand the deploy-often-and-rollback-if-there-is-a-problem strategy, but certain things like DB migrations and config changes are difficult to rollback, so doing a dry run in a staging environment seems like a good thing...