They quote an official here

[link to www.kcra.com]

"About 200 to 300 feet of the spillway itself has eroded away, it's just gone. And that's of course of great concern," DWR information officer Doug Carlson said. "We could let the water flow down the spillway, and continue to erode the base of it. That's preferable to allowing the water to get too high in the dam."

So it sounds like they are reluctant to use the emergency spout.