For the use case where you have a 'mongodump --oplog' backup and want to provision a new secondary from it, the steps are somewhat complex. You need to (a) mongorestore the data with --oplogReplay, (b) create an oplog, and (c) insert one document from the oplog dump in your backup into your new oplog.

Anyway, mongorestore might as well have a flag that does steps (b) and (c), provided that oplogReplay is being done (and probably also that --drop is supplied, since terrible things would happen otherwise with gaps in the oplog).