You fail to mention the biggest advantage the Leena event has over all others minus the Aerith event, that it's far less luck based to get what you actually want. I'm sure I'm not the only person who's had miserable luck grinding for things in the "best area to grind" for them. With collection events you can always see yourself making progress and getting closer to your objective.
With 1 or 2 orbs, sure. When you're talking in the range of 20-30 (let's face it, we'd have gone for all 10 earths, some darks, all 10 lightnings, assorted others) simple mathematics in terms of averages and standard deviations start to stack up. You have to be exponentially more unlucky having equally shitty drop rates for 20 orbs vs 2 orbs.
Does it feel better? It can. Just like rolling a 4-6 every time can feel better than rolling a 1-15 (numbers unrelated to anything and pulled out of my ass to make a point)
But yes, I agree, Lenna event is far easier than Heroics, and thus a good place to level a couple of lowbies in addition to grinding summon orbs and 3* orbs. For most 4*s though, the bosses (e.g. Mist Cave Elite) can just as easily be done by the B and C teams.
edit: Look I'm not trying to tell you how to play the game.
All I'm doing is putting out my own little analysis on the cost-efficiency of all the various methods in the game. Think of it as a game review. Except for events. Falk's Fabulous Event Rewards Review. You don't have to agree with it, just like you don't have to agree with IGN's game reviews (i'm not IGN tho
) and if it feels worth your time, especially if/when cpp starts getting more meaningful data on the exact rates and we can derive more accurate stam/orb for everything and compare just how different it is, by all means go for it.
Basically just from my years of farming useless trinkets in WoW and dozens of JRPG platinums, I've basically conditioned myself to think in bulk and always follow the most efficient average routines, even if they're more random.