When you're talking about heals per second w/ Dissipate, you have to factor in the fact that it gives you, for example, 3 600 potency heals off GCD.

Heal potency is increased by 20%, so Physick goes from 400 to 480 potency, at the cost of 300 potency w/ 3.0 (not 2.5)s cooldown from the fairy.

400 / 2.5 is 160 HPS.
480 / 2.5 is 192 HPS.
Fairy: 300 / 3.0 is 100 HPS.

So we see you lose some throughput w/o fairy.

However, you just gained 3 Aetherflow stacks.

You can only Lustrate 3 times. So we have to average out the numbers over the duration of Dissipation's buff.

Over 30 seconds, you use Lustrate thrice, spamming Physick 12 times.

((3 x 600) + (12 x 480)) / 30 = 252 HPS

That's 8 less potency per second than SCH + Fairy. How much of a difference do you think that makes? You gain the value of having 3 emergency heals. You can also use Indomitability or Sacred Soil. If the fight is long, resummoning will cost MP tho.

Feel free to correct any mistakes.