Wow so many replies oô.
I don't really think that it's mandatory for witches to change sides only for this use. Actually I find the idea kind of funny that while you're trying to get the witches in place, they throw potions at you.
An automatic netherwart farm would provide you enough netherwarts to make lots of the red nether bricks (which I find really hard at the moment).
Also I have seen many people who build automatic potion brewers.