I don't believe John's factually correct here. As a person who holds clearance, I can testify that it's simply not factual that the process is insecure or lacking. It's an extremely intrusive process - you give up virtually every piece of information about your life - your friends, your jobs, your travel, practically your entire history. Then you go through FBI interviews that cover everything about your past. Then they go to your neighbors, your family, your friends...practically everyone you know to verify your info and make a judgment of whether you're a trustworthy person. I'm sure they also comb your Facebook and emails these days, too. Re-upping the clearance typically involves even more scrutiny, since you've been exposed to sensitive information.
Put simply, it's not possible to derive motive or future disillusion from any questions they may want to ask. You can't look at who they voted for, political beliefs or any of that to truly determine a person's trustworthiness. There's simply no way to look at an individual, whom is probably trustworthy with sensitive information, and determine conclusively that sometime in the future they may become a whistleblower/leaker/whatever they want to call it.
The problem is simply this and it's a reversal of something we hear from people a lot. If they're not doing anything wrong, they have nothing to fear. But when you multiply questionable actions, of which could easily be interpreted to violate a person's morality, trample their personal rights and likely violate international law, we've created conditions for whistle blowers to come forward. The whistle blower concept revolves entirely around abuses of power - without which, there wouldn't be any motivation to reveal anything.