I couldn't imagine keeping a reef without xeniids. Aesthetically, they add mesmerizing movement. The more rapidly growing forms (eg., red sea xenia, and elongata) can be used as a nutrient export system by frequently cultivating them. Xeniids also tell you a lot about the health of your tank, sort of like an early warning system. Since they are sensitive to fluctuations in reef chemistry (e.g., ph, alkalinity, calcium levels etc.) they can signal a developing imbalance in water parameters. If one is concerned about overgrowth, slow growth xeniids can be obtained. I keep Bali Blue Heteroxenia and Xenia Umbellata on the rockwork of my sps reef tank to provide animation to the reef with no danger of overgrowth. Other xeniids can be kept away from the main rockwork by maintaining them on smaller rock "islands" on the substrate.