If you need to do careful recovery or forensics, a journal replay is sometimes not feasible. The contents of the log and rollback strategies can highly differ on ext depending on the Journal modes and (optional) checksumming. The solution can be to mount the filesystem without log access / replay. The journal can also be extracted quite reliably as long as the fs descriptors are still working.
This somewhat equals the ro mount behaviour of other filesystems, or i.e. the nolog fsck flag on some others.
Using the following mount option you can make sure that a read only mount is really a read-only mount.
mount -o ro,noload