Someone can tell me if I am wrong, but it was my understanding that the idea behind eliminating taxes on dividends was to encourage companies to pay out dividends to investors rather than encourage holding. I thought it was related to reducing some of the bad practices that led to cooking the books, Enron's, etc, where the small investor (stock holder) is the biggest loser. It made a lot of sense to me when it was explained.
Reagan the optimist would have explained it as a matter of principle -- the elimination of double taxation. The current crowd of pessimists and government control freaks see every team of corporate management as crooks. The government should make it very difficult for them to decide what to do with the money.