David Cay Johnston writes:

advocates of eliminating taxes on investments say there is no cost to the government because lowering taxes on such income encourages more investment

Look, capitalism doesn't say capital is better then work, it says capital is equivalent to work. Cutting taxes on investments is no better for the economy, in principle, than cutting taxes on labor (i.e. payroll and income taxes). To suggest otherwise is a toadying, cargo cult of capitalism, not the real thing.

In practice? I think it's not as good: but that's complicated, and for another time.