Assuming that I understood you correctly, and DRW give DC full benefit, while RS gets nothing from weapon damage, the new formula should be:
((0.514*x + 1133) * 1.05 * 1.10) * 2 = (((((x/14) * 3.3 + 29475 * 0.5) * 2) * 0.68 * 1.04 * 1.15) * 2 - Wolfram|Alpha
Which give a breakpoint at 108k AP. That should put it as the optimal use of RP when tanking multiple mobs, as you often cannot spent it fast enough, and you would pretty much always have more then 108k AP.
DRW isn't *quite* that terrible. Just almost that terrible.
Farther back in the topic there was some intrigue in the details. If you care to see the discussion, it starts here and mostly remains on that same page.
Basically what was determined is that the Rune Weapon--by "not copying our weapon damage"--rejects your equipped weapon's damage and substitutes its own. If you have a good weapon (read: any weapon better than a Cataclysm leveling green) then the Rune Weapon's damage will be lower than yours. If you have an ilvl 15 vendor white then the Rune Weapon will actually outdamage you.
Through testing of my own at the time I recall coming up with the phrase "Cataclysm leveling green" to describe it, so the weapon damage it works with should be roughly 2500. Sorry, logs from the time have expired and so has my sub, so I can't go check.
For clarity, its weapon damage does increase with our AP, exactly as weapon damage ought to.
Not to nitpick, just to explain. The difference is only 9k AP for higher DPE on DC than RS.
Edited by KiqJaq, 12 February 2014 - 11:04 PM.