Having good clear 'for Leona' does not mean she has good clear in comparison. I think you're just shirking the strengths of a lane-present Leona-- not to mention you're now two supports up. Leona can't really push (like Xin or Evelynn could manage) and she doesn't really have the kind of area or map command that the likes of Fiddlesticks or Nocturne have. She has high CC, but a lot of jg picks do-- and some of them, like Ivern, come with more than just the control.
I think the reason she doesn't see more use is that she's not an optimal pick. Unless your enemy's comp was particularly easily outed by roots/stuns, she just seems like a watered down option compared to lots of others. For what a JG has to accomplish, you could get a pick that would move your team further.
I don't know how viable it is or isn't, I'm just saying other picks will have a more fluid time with less finnicky workings, and might experience a more ready opportunity for comebacks if they fall behind. I could see Leona suffering from being behind, moreso than other melee picks, because she doesn't exactly scale to AD (so her individual buys are worth less, in theory).