Yes, I agree. Webscraper.io is awesome tool. Xpath selector would be a wonderful addition in time.
Many people argue CSS selectors can do all that Xpath selectors can do. But that's often because they are using JQuery along side CSS in their platform.
In pure CSS selectors, I often cannot chain-traverse up parent, ancestors, back down the siblings and others for very specific and tricky structures. Or pick a node by peeking deep into its descendents, but only select the node's sibling. Also CSS text manipulation for selection is limited.