It works, although I do it with Cho'gath. I think Leona's issue when being a defender is that she really doesn't have the damage to push out the minion lanes near a friendly capture point to put pressure on the enemy team.
With Cho, I have excellent minion clearing power to force enemies to defend against pushes, and I have the CC and burst damage to eliminate virtually any single person that tries to take the point I'm defending from me. I can also easily keep 2-3 people from channeling if I'm nearby with Vorpal Spikes AoE. If they try to dive me, I'm tanky enough to survive an initial burst and just CC them and let the tower do work.
When I get dived at tower as Leona by multiple champs, I CC them and then they run away and just continue poking away until I die. She doesn't have the damage alone to make enemies fear her. She absorbs a lot of damage but eventually they overwhelm you as full tank and 0 damage is no good solo. I just try to delay them as much as possible before I die. I don't even do enough damage to clear the minions so in many cases enemies actually captured the point while I was at like 80% health and just killed me afterwards as I couldn't do much to stop them or the minions. lol
I can see how Cho or even Amumu could perform better.