If anything, the capital gains tax cuts were actually bad for the stock market in one respect. Because of the enormous spread between the top income tax rates (38%+) and the top capital gains tax rates (20%), people had a financial incentive to ignore income-producing assets in favor of assets that may not produce income but had the potential for growth. Examples of the latter include not only growth stocks but real estate as well -- which is why, incidentally, the run-up in the stock market (espeically in growth stocks) was accompanied a boom in real estate.
. . . it's good for the economy to give a tax break for selling a stock that's a loser, and to penalize the taxpayer for selling a stock that's a winner?A capital-gains tax is a tax on anticipated future dividends. You only pay it if you sell the stock.