Well here goes then:
One of the redeeming factors about Togekiss is it's ability to tank hits and use a particular combo called "Paraflinch", which is basically abusing Serene Grace to continuously flinch a paralyzed opponent so that they don't attack half the time. This infinitely increases it's survival chance as a defensive Pokemon and often forces the opponent into tough spots.
The most abused attack here is Air Slash, which Togetic surprisingly doesn't get. Togetic also misses out on Aura Sphere, although it doesn't have the firepower needed to use it efficiently anyways. It's beside the point too since we're talking defensive here. But still, on the other hand, Togekiss gets every move Togetic can learn.
Now, don't get me wrong. Togetic does indeed, despite it's low HP, take less damage than Togekiss. But more HP means that when Togekiss heals itself using Sitrus or Lefties, it gets crucial amounts of HP back, it's chances of living are actually more than Togetic. Also, it also makes Togekiss a much better Wish-passer than Togetic.
This is self-explanatory. Togekiss because of it's item doesn't need to be wasting a turn to heal itself, while it's only a matter of time before Togetic corrodes down.
Now this is major. Togetic is slower than almost everything out there; 40 speed is not something to be proud of. It cannot even abuse Trick Room as it is not an offensive Pokemon. Togekiss, while not very fast either, outspeeds key Pokemon like Heatran, Tyranitar, Breloom and Bisharp, which allows it to
- flinch the opponent
- pass a Wish on it's turn, and heal the next Pokemon before dying
- Defog out hazards before dying
If they are faster, Togekiss can paralyze and then outspeed to flinch them.
TL;DR: Togekiss is a better defensive Pokemon than Eviolite Togetic because of it's ability to outspeed and flinch it's opponents (preventing an attack in the first place) and heal itself/it's team better.
Hope I helped!