I don't have a lot of time to spend on this, but quickly, here are some of the problems:
Currently an RTL user sees something like this:
[cid:image001.png@01CD11B8.3665AC60]
But when the URL gets bigger, a BIDI user had the most important information displaced. So the part where they look (right side) isn't where the important stuff is.
[cid:image002.png@01CD11B8.3665AC60]
Worse, clipping starts clipping it in the wrong direction:
[cid:image003.png@01CD11B8.3665AC60]
Clearly those are Latin examples, however the existing behavior with an Arabic domain and ASCII path would cause the same types of problems with respect to the most important (eg: domain) part of the IRI. I would probably even argue that in this case, where the browser UI language/system locale is Arabic, and the GUI is Bidi/Mirrored, that rendering ANY IRI with the parts ordered from right to left would be much more user friendly. That's a User Preference, or at least System Preference.
-Shawn
