Help with big pulls in newer content

[EaMett]

Well, you get the PI ogcd as bonus 150 potency, and in practice it does make a difference since you can definitely notice when it isn't there. Considering the Largesse frame things add up to about 120 potency per second (~17% of which is medica II). Also lets not forget the various dps damage clipping that may occur.
As for the gcd argument, medica II is placed during the pull phase, so you're running. It's wasted time anyways. Having to cure II later on will however cost you a gcd at the very least.
The mana is pretty insignificant given the management tool set whm has.
Keeping Swiftcast is however a valid point. But honestly, I would rather use swiftcast in this manner 10 times rather than hold onto it and only use it a couple of times.

I understand your argument and where you're coming from but in light of the above I still think increasing your passive HPS is preferable to holding onto your mana and swiftcast. Of course the above is situational. Sometimes swiftcast + holy is a good combo so you need to adjust accordingly but I find those instances increasingly rare.