This paper presents a theoretical and empirical case for the value of scenario-based assessment (SBA) in the measurement of students' written argumentation skills. First, we frame the problem in terms of creating a reasonably efficient method of evaluating written argumentation skills, including for students at relatively low levels of competency. We next present a proposed solution in the form of an SBA and lay out the design for such an assessment. We then describe the results of prior research done within our group using this design. Fourth, we present the results of two new analyses of prior data that extend our previous results. These analyses concern whether the test items behave in ways consistent with the learning progressions underlying the design, how items measuring reading and writing component skills relate to essay performance, how measures of transcription fluency and proficiency in oral and academic language relate to writing skill, and whether the scenario-based design affects the fluency and vocabulary used in an essay. Results suggest that students can be differentiated by learning progression level, with variance in writing scores accounted for by a combination of performance on earlier tasks in the scenario and automated linguistic features measuring general literacy skills. The SBA structure, with preliminary tasks leading up to the final written performance, appears to result in more fluent (and also more efficient) writing behavior, compared to students' performances when they write an essay in isolation.