Dodge hits diminishing returns faster than parry. You see people at 9 and 19 because they can reforge X amount of dodge into Y amount of Parry and get more parry than they were for dodge. The formula is the minimum, I also believe that the parry gap grows exponentially in comparison to dodge so you may have 7.5 and 9% but 9% and 19% might be closer to what the formula suggest.
If you could take 200 dodge and put it into 200 parry and it increased your parry by 1.5% but only reduced your dodge by .5% wouldn't you do that too?
The formula is all well and good if you don't compromise elsewhere to balance them. At the moment mastery is sooooo far and away better many simply ignore messing about with avoidance and just pimp mastery>hit>exp>haste.
Imo worrying about avoidance ratio's at the moment is purely academic. The ratio's were Theck's MOP beta work I believe, and even Pala's don't really care about it as things have turned out.