Okay, we have a thread on this very subject. And all of your points have been outlined in it.
As a DK tank you should break 20k dps, a good DK can almost do 30, I know our guild has logs of our DK easily pussing 26 to 27k on some bosses. 507 strength is about 126 parry rating or without DR about .71% parry. Which isn't as much a parry enchant could get you. Your missing out on 1.3% parry, which would not be subject to diminishing.
The reason DW is viable is because it is a slight avoidance gain AND about a 1.1% healing gain. The trade off being 8% less damage and 10% less threat.
[Today 06:48 PM] Ion:swimming in a natural body of water ISN'T acceptable...it's momentarily tolerable