DK’s need Runes. Haste increases ur Rune regen.
Without much Haste, u will find yourself short of Runes fairly quickly and unable to do much.
Also, Heals for Blood comes from many sources. Spreading Blood Plague is a small HoT, then Blood Drinker talent, u could also talent Rot & Wither. As u mentioned u can get Death Strike quicker. Crusader or Fallen Crusader also have a heal which proc quicker with Haste.
If u want to play ur DK like a melee class, u will struggle without a decent amount of Haste.
But if ur happy to sit behind the lines and be a plague spreader, then granted, u won’t need to worry about Haste.