You partly answered a question I had to the article: What makes the new algorithm that much faster than barriers? When I still send a "cache must be flushed" request to the device, even normal writes are stopped until the device flush it's cache, and it will take some time to fill the cache again to get good performance, during that another flush might incur...
Think of a simple file server, where 3 persons copy files to:
Person 1 copies his MP3 collection to the server.
Person 2 copies her HD-video collection to the server.
Person 3 runs a highly parallel intensive write database.
While for 1 and especially 2 there will be very low metadata operations, 3 generates lots of fsync's. From the article I read that a full cache flush would happen, suspending even the normal writes of person 1+2. Or am I misinterpreting that?