Very minor spoilers for A Dance With Dragons follow. Not really anything you'd be pissed at me for revealing, but better safe than sorry, right? Consider yourself warned.
I was IMing with Alyssa "No Mercy" Rosenberg today and we got to discussing a bit of a Song of Ice & Fire mystery. The Iron Throne is entirely in debt to the Iron Bank of Braavos. How can it be that House Lannister doesn't pay the most important debt in the Realm? Even with a multi-front war going, it's... out of character. There's no textual support for the proposition that Casterly Rock is strapped for gold.
One possible explanation: debt as war strategy. Keep the Iron Throne impoverished so potential rivals won't want a throne indebted to Braavos. Is Stannis, with his obsessive sense of (a version of) justice, really going to think it's worth it. Well, Stannis probably will. But still. I should mention that there's absolutely no textual support for this proposition, either.
Alternatively, could the debt be a weapon in Stannis' hand? If he knew about it, that is -- there's no textual evidence that he does. It's not like anyone loves the Lannisters. The great Westerosi houses that acquiesce to Lannister rule do so out of a paucity of other options and the surety of knowledge that finance is the strength of Casterly Rock. Stannis can make himself a more palatable option by going S&P on the Lannisters.
I guess the Lannisters could point out that it was Stannis' brother who actually bankrupted the realm. But would that really redound to their benefit? Blaming your predecessor typically works best if people hate your predecessor more than you.