So you are telling me that they thought about it enough to make you lose units and buildings when you have a negative balance, but somehow it is an exploit to let this happen? An exploit, my friend, is when you do something the AI can't do or doesn't expect or isn't programmed to handle. The AI, for the record, almost entirely runs their economies in the negative all the way (I have often watched the enemy players suddenly lose all their ports and be unable to trade with me while they were lagging behind in tech). Besides which, if you tried to run the whole game this way, in short order you would be out of libraries and universities and suffering a tech lag anyway, not to mention having lost all of your workers, artillery pieces, boats, scouts, explorers and then regular units...
No, the deficit spend is a valid tactic: far from being something that makes you feel good about winning, it usually just lets you breath a sigh of relief about staying alive in the game.
However I would welcome a RULING FROM CIVFANATICS on the subject...