I got the loss of ability plague once. it really surprised me what i saw my numbers do. It was pretty cool, my income was like -3000, my moral was 1% (not a typo) on almost all of my planets, all of my higher end ships where gone from the starbase list (as if they forgot some of the tech needed to build them). Luckily my research ability was high enough to get the cure in one week.
It was very late game and i controlled about 99% of the galaxy with only the Korath remaining. I checked and it was going to take them 51 weeks to get the cure (still good that they were researching it in the first place, smart AI). So they were all but useless. The best part was when my transports got to their planets; their soldiering was about 0. And their moral was around 30%. So using information warfare, for free with the evil improvemnet builing i had, i actually ended the invasion with MORE people that i had on the transposts. It was good clean fun. Surrender came soon after.
With the population plague, i do sell it but i wait,,, and wait ... and wait... then i let my mini-soldiers deliver the cure. Ok i do sell it to some but only fter they lose a lot of people.