Ultimates/Abilities through Zhonya's

Why? I get the advantage of using ultimates and then activating so you don't take damage but still deal it. But first I'd like to point out my problem with Swain. From what I could tell, he still gets health back from his ultimate even after popping . Using prevents anything from happening to the champ who used it, so it should make sense that Swain shouldn't get health back while in . And If I'm wrong about that and someone can show me an example of him not getting health back while in then I will admit that I'm wrong and we can move on.

But really, why allow ultimates/abilities to persist when is used? It only fits a certain niche of champions that can actually utilize it (ie Fiddles, Swain, Kennen, insert champ here that I missed). I would just like an explanation as to why that kind of mechanic was put into use. It stands to reason that it shouldn't work because the champ using it can't do anything during and no enemies can do anything to them either. So why should they be able to have an ultimate/ability persist when they're doing nothing?