Write barriers aren't just for durability - they also can even out major latency spikes when bloated buffers ultimately must be flushed. Database, filesystem, RAID, device, flash management and individual chips all become saturated at some point. Managing performance at the database engine layer gives you visibility into the issue at a high level as opposed to rolling merrily along until commits start taking 2000 ms. As an example, ZFS is terrifyingly unpredictable at high loads.