Bad example. The sole proprietorship actually ends up being an argument in favor of higher marginal tax rates.
Remember that businesses, including sole proprietorships, pay taxes net of expenses, including employee salary as well as equipment purchases and whatnot. Raising the marginal tax rate makes it less profitable for already profitable companies (including sole proprietorships) to hold cash positions because they need to pay more money on that; they're better off investing the money into the business, for example, by buying more equipment or hiring more people.
Buying equipment increases consumer demand and stimulates the economy. Hiring more people creates jobs and stimulates the economy. Holding a cash position, on the other hand, simply creates more money in the bank to just sit there (that's one of the current problems with the economy; money in the bank is indeed "just sitting there" because banks themselves are sitting on cash positions or buying Treasuries instead of lending it out -- which would also stimulate the economy, but it's not happening.)
So, basically, raising the marginal tax rates would force successful small business to create jobs and increase demand, which would in turn help marginal small businesses become more successful (because of the increased demand).
That's an argument for increased marginal tax rates.