Why does citizenship really matter (other than creating a mechanism to be taxed)

Came across the following article today:

Thomas Mulcair defends dual citizenship: Quebec MP and NDP leadership hopeful holds Canadian and French citizenship

Former Canadian Prime Minister John Turner is  dual citizen. So too are Canadian politicians: Stephane Dion, Thomas Mulclair, David Alward, and (according to the Government of Canada site) Green Party Leader and MP Elizabeth May was born in the U.S. Why does citizenship matter in the first place? If so, why would anybody remain a citizen of a country that treats its citizens as property to be taxed. Can anybody afford the cost of U.S. citizenship? Interesting article in Account Today about the number of U.S. citizens renouncing:

https://twitter.com/#!/renounceus/status/159735066036146176

https://twitter.com/#!/renounceus/status/159121532596125697