Keep in mind, another reason people don't tend to like Dissipation is for thematic reasons. Many players chose to play Scholar because of the Fairy. They enjoy playing a pet-class mixed with a healer. Getting rid of that pet, even if it's only for a short while, makes the Job less fun for those players. Dissipation gets rid of your pet, has anti-synergy with Fey Union, has non-intuitive interactions with your aetherflow abilities (since it doesn't boost their healing), and lastly requires you to spend both a fair bit of mana and a use of swift-cast to get back to full capacity once its over. It hurts not only the "class fantasy" of scholar, but also has a lot of major downsides.
It's more useful than it immediately seems, but it's still not an especially fun ability to use, especially as a level 60 capstone. Obviously, your mileage may vary on those points. Some players will certainly enjoy Dissipation, but many do not.
