Credit gives leverage to help improve ROI, but also brings risk. The investment banks are playing a dangerous game - they could make big profits out of their gyrations during the crisis, or they could go under. Buffett keeps lots of cash around to reduce that risk, which means he sleeps better and can take advantage of times like this. Berkshire will very likely still be there, and will be making pretty good money regardless. So - I think a positive balance is better, but you can't just leave it in the bank. If you use it well (and add debt where you're more sure of the outcome), you should do better over a wider range of situations than those who primarily rely on borrowing.
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