Each of the services has a "debt load" at which they're required to suspend your security clearance.
I think it's around 40-45 percent, but the idea is that if a troop has too high a debt load, he'll be tempted to sell equipment or information to People Not Intended To Have It. In the 1980s we rooted a LOT of spies out of the American military, and almost all of them were motivated to sell out their country by the Almighty Buck.
Check out
http://rf-web.tamu.edu/security/SECGUIDE/Treason/Numbers.htm and you'll see that money was a motivator in 69 percent (and the sole motivator in 56 percent) of all of the spy cases cracked in the 1980s. (You'll be pleased to know that the 1980s were called the Decade of the Spy because we busted a massive amount of spies in the 1980s, and almost all of the spies we busted in the 1990s were spying in the 1980s and were unmasked after the Cold War ended. Also remember what president was in power in the 1980s--yup, most of the spies the United States has ever unmasked were working during the Reagan Administration.) It stands to reason that if you've got such an obvious indicator of spying potential, you'll take steps to alleviate it, which is why they pull clearances when debt load gets too high.
On the modern battlefield, you MUST have a security clearance. Everyone out there has to be able to talk on the radio, and you need a security clearance just to pick up the handset and squeeze the button because the Signal Operating Instructions, the little book that lists the callsigns for everyone in your unit, is a classified document. Maps can be classified. A lot of weapons are classified--Stingers, for one. The challenge-and-password so you can reenter friendly lines are classified because they come out of the Signal Operating Instructions. You can't fight a war if you're not cleared.