As a Death Knight, Agility is for all intents and purposes a useless stat. All it does is grant you a tiny increase in chance to score critical strikes (and I do mean tiny). So, there is never ever (ever!) a time to prefer an item with Agility over one with Strength.
All items in the game (with a few outlandish exceptions) have two primary stats, and two secondary stats. One of the primary stats is always Stamina, and the second is either Strength, Agility, or Intellect. For this reason, even if you equip an item with Agility, you will still notice a change in your health (affected by your Stamina). The secondary stats are either Critical Strike Rating, Haste Rating, Mastery Rating or Spirit, so again, you can equip an item with Agility and Critical Strike Rating, which would greatly boost your critical strike chance - although this is largely unrelated to the Agility on the item.
If someone is criticizing you for using Agility gear at maximum level (90 in this case), they are correct. You should be using Strength gear at all times. That said, while leveling up, questing gear often has weird itemizations, and your DPS is affected far more by your usage of abilities (and by your level) than by your stats