> And what if it's something you need to use
  > once in while? Why a throwaway email address?

Then I use it once in a while, but I'm prepared to abandon the email address. "Throw away" doesn't mean that I use it once and can't use it again. If I find that the service is useful and trustworthy then I continue to use that address and promote it into full service.

  >> "I don't trust Twitter"

  > You don't try, or you just leave after finding
  > out that the app can follow/post tweets?

I don't understand your question, but I'll try to guess at what you might mean. To put it simply, I don't trust Twitter, nor apps that insist that use Twitter as an authentication method. They end up being able to post as me, or read my tweets, or read my DMs, or whatever. I won't allow that, and I don't have time to chase down what they may or may not do.