Now that I've thought it over while I still don't like the idea of corps spreading randomly I can see possibilities in forcing one of your corps on your rivals. For example you can gift a rival Civ a resource that a corp needs, then you can start spamming executives and spreading your corp around. They might mess up their economy due to all of the increased maintenance your rival will have to pay while making you at the same time. If they try to change their civic to State Property or Mercantilism you can use your spies to change it back (or even use the UN to force Environmentalism on everybody). I'm surprised they didn't include an economic victory as well.