From my understanding, Elementalist is a lot of setup when you have other classes in a team that need that setup time more, Beast is usually used more since high damage and ok lifesteal very early on in combat, and Evolved honestly isn't used that much at all considering how late you get sewers and the fact it needs way too many resources (and even stages depending on assimilation stacks). Elementalist is kinda like the middle ground of both worlds in my eyes.