As I've mentioned, I've also been playing a healy-Priest alt on the side. Even though this lesson was learned on a non-DK, it applies to us all.
If you're levelling and you learned Dual Talent Specialization before level 80, when you train your new spells from levelling up, it will not update your non-active spec's actionbars. I just realized that after healing heroics, Naxx10, Naxx25, OS10, OS25 and Ulduar25 that I've been running around with my healing spells two ranks below what I could have had them at, effectively halving my healing.
So remember, check your spelling, particularly right after you ding 80! No reason to be running around with a half-power Unholy Blight.
As to why you wouldn't want to use downranked spells, it has been rather a while since downranking was beneficial; all spells cost the same regardless of rank. The only difference is diminished effect.