Based on your financial expertise, provide your response or viewpoint on the given financial question or topic. The response format is open.

Why pay estimated taxes?
In addition to the other answers, which cover the risks of what is essentially leveraged investing, I'd like to point out that the 2.6% penalty is a flat rate. If you are responsible for withholding your own taxes then you are paying tax four times a year. So any underpayment on your first quarterly tax payment will have much more time to accrue in the stock market than your last payment, although each underpayment will be penalized by the 2.6%. It may make sense for someone to make full payments on later payments but underpay on earlier ones.