I do, and I don't agree with that. She isn't meant to be a duelist, but that can always change.
I mean people have done solo mids with Leona, but I don't think it's something that works often enough for it to be viable.
So lets say for curiosity she does have the ability to proc it herself, on everything. Now she can go anywhere. Now what? Do you throw her top? Why? Just because you can? How would that even be viable againts normal Top laners, verus Olaf, Irelia, Singed, etc.
Do you start roaming if your ahead? or just tower camp if you get behind? That offers little to a team composition, and is doing more harm than good.
Now lets say for example you send Leona mid. Sure she can have a hard time against most normal AP mids, especially if they have ranged basic attacks. You are going to farm with W and E, and stun with Q if anything happens. You might have the possibility to roam. You also have great opportunity to lock down a champion, but i don't think Leona would have enough burst damage to finish anyone off. The AP scaling isn't meant for that. Again, I think she can get outplayed a lot by most AP mids.
Now lets say she can jungle. She has great ability to clear with the AoE, and her W gives her some more resistances. She is going to be heavily mana dependent though. Her mana costs and cooldown may force one to use good timing on skills and procing the passive.
Now what can she do? She can start ganks. At level 3 she has very high presence. Her range on Zenith Blade is very good. You have to land the ability though, to have a chance of getting a successful gank. Otherwise you need to sneak in from behind if that fails, and walk up for the stun. Hope the enemy burns a flash, and comeback in a few seconds. The only downside I can see is her mobility. You would need to spec masteries or runes a little bit, and this can help with traveling between camps. Speed will also help geting to a lane faster, so there is less time wasted in walking. You also don't want to be spending too much time camping a lane, as with any jungler.