They've only ever had as much Evasion and Defense as they do now, even in 1.x. The only thing lost was elemental resistance. (And I'm completely fine with its disappearance as it only ever diminished an already 2 to 3 button rotation or locked out stance choices.)
When enemies have Defense or partial/granular evasion, the assumption is that the higher the enemy level, the less damage you'd deal, assuming the same nominal damage, because they're mitigating a larger amount of it. It's one of the ways many MMOs curtail the appearance of damage growth over increasing levels of strength, and sometimes even to decrease the damage spread against mobs of increasingly lower level than oneself, thus making it easier for higher levels to contribute in a more squished manner in lower level content.
However, it also has a second benefit, as when there's a way to remove Defense or (granular) Evasion that is normally present, you're essentially allowing for bonus damage. Any form of CC which prevents Evasion (e.g. bind or better) or Defense (e.g. stun or better, where sleep could be considered stronger than stun in that it can diminish resistances, Defense, etc.), then offers a potentially significant advantage to the spell which breaks the effect.
You'll tend to find, though, that when such is permitted, more than just the duration of CCs see diminishing returns--so does their potency and/or chance.
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My largest issue with Sleep, and CC in general, though, isn't that they don't provide potential tactical damage bonuses, and therefore become nonviable or bloated keys. It's that they have no place, tactically, in the majority of combat. There is no granular resisted potency to allow its use in places where its full potency would be too strong, thus limiting opportunity, nor any damage bonus to the then-vulnerable enemy or enemies, thus limiting value, but more importantly... there's no reason to use the CC in the first place, thus showing... a conflict of vision.
I feel like the game could and should make better use of an element of preemptive caution/awareness and strategic and geometric thinking. I don't want it to be constant, but just as I'd like for the game to allow for more variance in rate and difficulty in a given piece of content, I'd like to be able to see iterations among many where CC feels very much a integral tool, cohesive with the rest. And the current designs just don't allow for that, whether by the way CC is coded or the challenges it'd be used against.