You have to have money to make money. Marginal taxes redistribute at least a bit of that wealth, which in the end puts more money/services in the hands of less advantaged people. Obviously those in power will always fight to remain that way, but any edge we can get the less privileged dulls the edges on the other end of the spectrum.
The higher tax rates are a must to ensure that people with wealth and high income are paying their fair share -- paying back to a system that has given them so much and allowed them to make their money and keep it growing. However, it will not really reduce their power, as that is a more complicated issue of social status and connection. The higher rates are just about bringing a little more fairness into the picture.
The powerful people are going to be powerful no matter what. It doesn't matter if they are taxed at a higher rate or not. That's the point of powerful friends, isn't it? You can get them to do whatever it is you need. Yes, money helps, but it is not the only bit of the power equation.
Even though marginal tax rates are generally progressive, high income people are still going to have more money than folks in lower tax brackets. The rich people can also avoid some of the taxes with creative way to find deduction and tax credits. The wealthy individuals and families will not have less power.