my point is that enforcing a barrier through all these layers can be expensive (on a multi-disk array you would need to make sure that one disk has completed it's work before submitting the write to the next disk)
this isn't always needed, so don't try to do it for every write (and I've straced a lot of code that does lots of wuite() calls)
do it when the programmer says that it's important. 99+% of the time it won't be (the result is not significantly more usable after a crash with part of the file if it's not all there, or this really is performance sensitive enought to risk it)
you would be amazed at the amount of risk that people are willing to take to get performance. talk to the database gurus at MySQL or postgres about the number of people they see disabling f*sync on production databases in the name of speed.