The write-barrier ONLY serves a purpose for preserving IO order after a crash. For the processes using the file before the crash, the kernel is already providing the abstraction of ordered IO, and the on-disk ordering does not matter at all.
So, for many temporary files which will be purged/ignored on crash recovery, the write ordering doesn't matter at all. The only reason the files ever need to go to disk is for backing store in case the block cache is flushed.
So while I agree it would be nice if write-barriers could be on by default, this might need to be a configurable policy if it cannot be proven to have negligible performance penalty for all cases. And if it can ever be disabled as the default, then applications need a way to request it explicitly to override a default "fast but unsafe" treatment.