So, quickly, on the design philosophy at play here (you can feel free to disagree, but this is what I feel):
We want the Thief to be the class that most “slips through your fingers”. Other classes like the Ele and Mes have some of it too (and rightly so), but if anyone is escaping a fight, that should be a slippery Thief.
The Necro, on the other hand, should be the dude you CANNOT get away from. It’s an attrition based class, so the idea is that if you lock horns w/ a necro, know what you’re getting into: you’re fighting a class that’s built for attrition. It can dot you, dps you, rip your boons, and severely hinder your movement. AND it also has the ability to soak up a lot of damage. So the longer the fight goes, the stronger the Necro should get. That’s the idea behind Death Shroud, but little escape ability.
So we don’t want that attrition class to also have great ways to escape. This used to be the case, and we felt the Necro was just too strong if it was great at attrition as well as movement/escape. We wanted the Necro to be more about attrition, and for other classes (Thief, Ele, Mes) to be more about escaping and mobility.
Hope that makes sense.