Actually, the in-line overhand knot is easier to tie and untie than any of the other methods of joining two rappel ropes...and it pulls much more easily than a figure eight or fisherman's knot. And the evidence that it is safe is not merely anectdotal. In his 2002 book "Knots for Climbers" Craig Luebben, an AMGA certified guide, lists the knot among those used to join rappel ropes. His exact words are: "The overhand knot is sufficient for body-weight applications like rappeling, but should never be used where higher forces are possible." Plus, the testing data cited earlier in this post makes it clear that, under normal rappeling, it is almost impossible that sufficient force could be generated to make the knot roll even once.
The reasons to use the overhand knot are: easier to tie and untie, and much less likely to hang up when being pulled. Anyone who has had to jug up a stuck rope will especially appreciate the value of the last adavntage!