Communication skill is a proxy for critical thinking skill - if you can't communicate effectively, your critical thinking skill is either going to be undervalued or underutilized.

For the lack of understanding, communication inputs are misinterpreted, ignored or incorrectly responded to. For the lack of clarity, precision and understandability, communication outputs suffer the same fate. And, as we've learned in networking, inefficiencies in a system (such as the engineer whose emails make no sense, when he/she bothers to reply at all) get routed around.