I was just reading about the Scout, an all-solid rocket, and thought of Rocket Lab's Electron, which seems to have a similar mission of deploying small payloads to orbit. I understand the engines to be the most expensive part of a rocket (especially the turbopumps), and the Electron has nine(!) Rutherford engines on the first stage. Since Rocket Lab has no plans for reusable launchers, it just seemed that a solid first-stage, with no plumbing or pumps, would be a lot cheaper.
But I can see the value of liquids for upper stages because they are more controllable, making it easier to fine-tune an orbit. And the Electron second stage only has one engine, anyway.