Go back and look at the debates from that election- Bush wanted to "return the surplus to the people."
First, less than .1% of small businesses will ever make enough to provide $500,000 a year. If you are in the top 10% of small businesses in a good industry, maybe you are pulling $150k a year. Hence, taking out a million dollar loan to start a small business is probably not wise. If you are starting a business that requires large capital on hand (an insurance company, maybe?) then you should look for venture capital to make up this balance. A loan would probably kill you.
That said, let's say you have a history of success and a sure fire business plan. You take out your loan, and are instantly making 500k. Of course its worth it at 91% marginal taxes. Ignoring medicare and FICA (the interested reader can crunch those numbers), your total tax liability (assuming .91% for income over 250k) is about 300,000. You still pocket $200,000 a year.
You are also (in your scenario) the sole owner of what must be a tremendously successful company (if you can pay yourself 500k a year), which means you can probably sell it for a substantial profit, probably tens of millions, if you chose- which you would pay capital gains on, not income tax. So yes, of course its a good investment.
Your question implies you don't understand what a marginal tax rate is.