Why TBN is a powerful but poorly designed ability.

[SyzzleSpark]

snip

DA+DM being occasionally numerically better doesn't change the fact that you're still out 2400 mana with no bonus BG to show for it. The margin is too thin to make a DPS loss worth it.

The point is that TBN+DM will cost you a fraction of the resources while providing similar if not better mitigation, thus rendering DA+DM worse overall.