[donatj]

It affects float direction. Imagine the browser detecting Arabic text and suddenly all your float lefts are floating right. Switching it from one side to the other will completely break a lot of layouts, particularly older ones.

[marcosdumay]

Good thing that there is no reason anymore to use float to hack page styles, instead of floating objects within your text. So that switching the direction is the correct thing to do with reasonably placed float out there.