This could certainly be a boondoggle, but reducing flow through a dam is limited to storing excess power at the natural flow rate of the river, whereas there is

This could certainly be a boondoggle, but reducing flow through a dam is limited to storing excess power at the natural flow rate of the river, whereas there is no limit in principle to how fast you could pump water from a big reservoir below a dam to a big reservoir above the dam, or how fast you could draw energy from greatly increased flow through your generators. So pumped hydro storage isn't an inherently silly idea.