When you subscribe to Yahoo or Hotmail, you create an account with them and you knowingly use their computers. But if someone else offers you to use its computers, they should have no business to know your account details on the Yahoo/Hotmail computers. It is like using a banking application created by a third party, to access your bank account.
I dont care. That's not going to get me a pat on the back for my security consciousness, but I still trust meebo. If you do not, then good enough, but some people just might go for it. Its the degree of security and trust that matters.