Ah... no, it means she is now a White Court vampire who can stop and start their hunger with a skill comparable to the most experienced of elders though artificial means. Isabela has fed enough off Sarah Greene to awaken her hunger it's just that she has no native control over it, all of her control comes from Mercy in Servitude.
Ok, so, to be clear: right now Isabella is effectively (meaning her abilities are the same, her powers are the same, her hunger is the same, her lifespan is the same) a normal fully fledged Skavis vampire. She has perfect control of her hunger, rivaling or even exceeding those of elders of her family. She can feed, and use the energy she obtained via feeding to fuel her supernatural abilities as normal. She can store the energy she obtained via feeding for later use, like other whampires can.
However, this state of control is artificial, fully imposed by MiS. No matter how long she spends under MiS, or how much she feeds, she doesn't actually get any experience and ability to control her hunger. The moment she leaves the effects of MiS, her control reverts to zero, and remains such until she feeds on someone to death.
The question is: what happens to the energy she has stored while under MiS effects? Let's assume she spends the next year or so slowly feeding and not using her abilities. She fully charges her hunger while under MiS to the point she can't store any more energy. Where does it go when MiS expires? Does it explode out of her? Because Dresden Files actually pays at least a bit of respect to conservation of energy.
The one person in this equation is her, I don't care about the social disruption among the skavis clan in this context. It's a toxic environment where being popular and influential involves being an asshole, so we should probably be trying to get her exposure to other people anyway.
Strategically, there's also Red Half-vampires. Who transform much more when feeding on humans for the first time (because it universally and absolutely involves a murder), suffer more, and are more innocent in that they didn't get a choice to awaken their hunger. And there are also ghouls (do they need human flesh when first going through super-puberty?).
Even leaving them aside completely - you said it yourself: skavis and other whampire clans are a very toxic environment, essentially murder cults. If we can extract people from them, and reform them, I would argue that it's a moral imperative to at least attempt so, as long as it doesn't hinder our other actions.
Doesn't this just support my point? Not being a murderer is significant for this purpose, but we should also be thinking of what this flavor of primordial essence brings to the table.
Which is why I am ok with waiting until we get our kingdom, as it changes the essence flavor from "hell of burrowing maggots" to "Primordial Molly's world body".
This same discussion is just going to happen again. The rationale against getting it will be the same, and even if we did have it using it on Isabella would be a very bad idea.
It's kind of a dick move to come out of a contentious vote and immediately go into "we'll just do this again until the opposition gives up".
That's how democracy works, though? If you are campaigning for something, and lose the election/vote, you don't just abandon the cause completely. You continue to campaign for it, and vote for it in the next election / vote. Until you win, or the circumstances change and you abandon the cause.
By not choosing to buy MMP this turn, we don't bar ourselves from buying it next turn, or the turn after the next. Same as with other charms, abilities, backgrounds, etc. Not unless we make an explicit vote to prevent ourselves from buying certain charms in the future, and I don't see it happening.
EDIT:
So this isn't actually a problem after all; she isn't missing out on anything by being stuck like this, as long as she doesn't betray us.
Mechanically, and psychologically, the bondage we are putting her in by this is likely much harsher than the one created by MMP.
