Disk Uses More Space Than Size of Files
Few programs use it and they shouldn't rely on the
meta data being there since moving
the file from NTFS to FAT destroys the alternate streams.
One version of Kaspersky, used alternate data streams for
tracking files it was monitoring. So it has been used.
In that case, a small percentage of Kaspersky users
ended up with problems somehow related to alternate streams,
and the usage of that was abandoned in the next version.
On modern Windows, alternate data streams are
supposed to be used for tracking whether you downloaded
an executable. And then you can be appropriately
warned, when attempting to execute something that
came into the machine via downloading. There are
recipes around for removing the metadata, so
Windows will stop whining about that particular file.