I'm not sure that i understand what is it for.
I tried it in a mixed-directionality environment and it seems to work correctly with and without this line. If i work in a Hebrew wiki with English interface and add a period to the word "edit", it appears on the wrong end even without this line. If i add "direction: ltr", then it appears correctly, and it works even without "unicode-bidi: embed".
Hmm, I'll revert this because it was a fix for r93386 which was reverted.