By forcing the cure down Katherine's throat, it may seem that may have won, but by not killing her she now has to protect Katherine while Katherine is trying to make her life a living hell. This will make season interesting.
Well the writers will come up with reason because if Katherine can become a vampire the very next episode then turning Katherine human would be pointless.
My guess is that there is no gauruntee that she could be turned again or maybe she will try to cut a deal with Klaus with her doppelganger blood. Who knows what writers will come uo with.
You just mentioned two "points":
- First it used up the "cure", making it un-avalable to Silas
-Gives Klaus the hybrid blood.
But you are right , let's see what the writers come up with!
What do you think?