Doing mirroring in the file system would also allow mirroring of metadata and files that
warrant it. That big video temporary file could be striped on the other hand - and later
perhaps converted to mirrored with some kind of chattr command.
Cutting out the layering also enables better layout of data on the disk. When writing, a
single small write would be kept on the same disk, and not straddle a stripe boundary which
would lead to unneeded extra IOs. One could even write a file as raid5 or raid6 while other
files on the same FS are mirrored or striped, or just on a single device.
However, writing multiple files each to its own disk, and keeping track of the parity
independently is much more complex. That would be very good for read performance (only one
head moves for each file read), and raid 6 is better at protecting data than mirroring.