Thank you for pointing out
http://www.freebsd.org/doc/en_US.ISO8859-1/books/arch-handbook/driverbasics-block.html

And yet, FreeBSD defaults to turning on the write cache in disks.

> This caching makes block-devices almost unusable

I find them quite usable.

> depriving the application of the ability to know the exact disk contents
> at any one instant in time

Most applications don't need to know.  All the world is not a database.

> This makes predictable and reliable crash recovery of on-disk data structures
> (filesystems, databases etc.) impossible.

I was under the impression that filesystems had the ability to force
the order of writes when they needed to?  Am I misremembering?
Is this a problem in NetBSD?

Databases can use raw partitions.

> For this reason, no serious applications rely on block devices

Only for wierd definitions of "serious".