Oh boy, more people who never use Dissipate. I'm not sure of some people's experiences or if they don't play Scholar in extreme/savage end-game. It does have a use, but if you only do dungeons/casual content you probably will never find a situation where you need those stacks unless stuff really hits the fan. I'm a stanch defender of this skill because there are good uses, albeit niche but they can save a raid too.
When Sephirot Extreme was prime-time content, there was a lot of damage to heal at i210+ levels. The add phase was pretty lethal if DPS wasn't so controlled but this was a great place to use up stacks and Dissipate, your MP wasn't very high if you DPS quite a bit by then. But you'd replenish in no-time and you'd get better shields for the add damage which helps the other healer not have to heal so much. By the time the phase was over you have downtime to re-summon manually.
Fairy Swaps. There are instances where one fairy may not be the most optimal use for the entire fight. Knowing this advantage separates a really good Scholar from the rest. With this in-mind you can take advantage of Dissipate to get stacks, more potent heals, or more DPS before swapping to the other fairy. Or instead of just burning Swiftcast/Summon. Places like A6 Savage where Selene was best to have out for the first two robots to push DPS, and have a Silence if-needed. Then before Brawlers add phase you can Dissipate to get better shield and get Eos out for the rest of the fight.
Or more current Savage, A11S having Selene out to start until Limit Cut to push DPS since AoE damage is limited, and you have Selene's Fey Caress so don't have to wipe the raid just because of deaths. If no death, just Dissipate during the phase and re-summon. Or A12S during the two adds is another prime spot, you can even pop Whispering Dawn to get regens out and Dissipate after she casts and you still have regens up with 3 more stacks. Can just re-summon during the ultimate downtime and/or do the Selene Fey Wind/Eos swap like a real pull.
These are just a few examples. Knowing that you can pop fairy cooldowns and then Dissipate is something to keep in-mind with the Scholar toolkit. Eos doesn't have to be out for cooldowns like Whispering Dawn to continue to get it's full effects. She just has to be able to cast before she is eaten.