After some googling I found something:
http://plover.net/~bonds/genderedlanguage.html
"... The problem is that in the English language, the "traditional" gender-neutral form -- "his" -- happens to be the same as the masculine form. And to say or imply that an arbitrary or unknown person must be male, as those who use "his" are claimed to do, is discriminatory... "
I read that the legal doublespeak comes from English roots. When the Anglos and Saxons had to start getting along, their lawyers had to use a word from both languages. Big [citation needed] here, so I hope someone obliges. I'd love to know the truth.
But back to "his or her." I often use simply "her," to be both consise and politically correct. ;)