Whether you think Bradley Manning is a hero or a traitor or just some guy who you're sick of hearing about on the news, it's clear the harsh sentence is meant to send a message to future leakers, even if—especially if?—they leak to the press, and even if—especially if?—they do it with the goal of exposing wrong-doing, as Bradley Manning's own words and actions from the time clearly show. The sentence is five years longer than that handed down in a 1989 case in which Army Specialist Michael Peri was convicted of supplying intelligence information to Soviet spies.
“There’s value in deterrence,” prosecutor Capt. Joe Morrow told the court earlier this week. “This court must send a message to any soldier contemplating compromising national security information.”
It's simple: The harsher the penalties for crimes committed in the act of whistle-blowing, and the more zealously whistle-blowers are pursued, the fewer whistle-blowers there will be. As for the importance of whistle-blowers to democracy, Harvard professor Archon Fung made the case in the Boston Review better than I can:
We will always need whistle-blowers because democratic institutions cannot be perfect. Inherent in the nature of discretionary power is that politicians and policy makers inevitably act—sometimes legally and sometimes not—in ways are that are unjust or abuse the public’s trust. In the face of such wrong, each citizen must come to his or her own considered judgment about whether a law (that black people must sit at the back of the bus), a policy (to keep secret deliberations that the country is engaged in a war that it cannot win), or an order (to abuse military prisoners) is so wrong that she should break the law or otherwise disobey authority to bring public attention.