I'm not saying Leona is good right now because AS LONG AS 
ARDENT CENSER IS IN THE GAME...Leona will pretty much be trash in comparison to Janna, Sona, Lulu, Soraka, etc. I'm just saying that they needed to somehow change Ardent Censer as opposed to buffing Leona.(Or at least do that first and then see if Leona STILL needs a buff).
EDIT: Honestly pretty much the same situation as Ezreal(the difference though is that they are releasing a skin for him)
Lol.... it is not a buff. I don't know what the balance team at Riot was thinking, but the changes demonstrate a clear lack of understanding of Leona's problems.
The Q changes.... Maxing Q was already the path so by level 7+ the change is actually a nerf (+1 second to cooldown). On the other hand, it does give Leona stronger level 2 all in potential. But if your win condition as a champion is a level 2 all in coin flip.... well... Most of the time you want to wait for 3 anyway...
The sunlight damage. Again a fundamental misunderstanding of Leona's problems. A small buff to her early all in damage. +39 magic damage late game is just laughable. That is before resistances...
The problem with Leona is not her early or mid-game. It is late game scaling. Instead of adding more pathetic magic damage, which tanks (or anybody with 1 mr item) will just shrug off, make sunlight true damage. Then it would actually scale properly, and actually contribute something to lategame teamfights.
Another possibility would be to give her a unique interaction with sunfire cape, where it does true damage for Leona. But yeah, she scaled to nowhere before.... she still scales to nowhere. Zzzz.....
Sona Main (Low Elo though) and I struggle vs any competent Leo, and forget it if they are good with the champ.
Leo is my secondary and I was surprised by the buff to be sure.
I can see both sides as to why to buff and not buff her, as although she's a pretty useful hard-engage pick she's seen literally no use in competitive for a while (I'll have to check, but I'm pretty sure the last team to use her in one of the 4 major regions was one of the LPL teams at worlds last year, only after the opposing team banned literally every meta tank support). If they're doing this because they want to see more support diversity, I'd much rather see them just turn ardent into a piece of an "Exodia" thing, in which if you gather 4 of the shielding/healing buff items combined they form something ridiculously good while having the seperate pieces relatively weak (perhaps with them stacking on eachother and doubling the effects of shielding/healing bonuses).
riot balance team don't know what the fuck they are doing half the time
lmaoooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo
leona shits on defensive mage supports. all over em.
If you are in Bronze, she can be good because players just don't know or will not respect her engage potential. But even by high silver, most supports with half a brain will just position correctly vs her and gg.
Also, if someone on the enemy team picks Leona before you as support, you pick Taric and win the game at champ select.
If you want to play as Sona vs Leo, just have a tanky rune page (armor reds and/or quints). Resident sleeper in lane. Win every teamfight. GG.
My best guess is it has something to do with the intended scaling of the game's current meta? Create a potential situation where she can be more active in early and mid game fights with less imposed risk as something designed to be a tank.
