I don't see why it matters whether or not he uses cash or stock. If he had bought dexter using cash, the investors could have turned around and used that cash to purchase berkshire stock - making the deal equivalent to buying with stock. The fact that the stock went up over time would have been true in either case. I think that the comment about it being a mistake to have used stock instead of cash, and that it was compounded, was simply to make for a better story.