If Section 31 had ever intended to use the cure why didn't they let it known that they had it instead of letting Earth and Betazed get attacked? And for that matter why hide the fact that they had the cure in the first place.
Let's see, likely Admiral Ross (as a member of S31) knew of the sickness and the cure. And S31 (at least claimed) that the President's inner circle included a S31 member.
Initially the sickness had to be kept a secret from the Founders so they wouldn't contain the spread, perhaps quarantine their own people who traveled to and from the alpha quad.
Even after knowledge of the sickness and the cure moved outside of S31, the Federation Council (who might have known about it from the start) made the decision to withhold it until they thought it wise to release it to the Founders.
At that point, it became a political decision.