It is sometimes difficult to obtain mission focus from untrained subject matter experts, even in facilitated workshops. Merely getting consensus on the mission can be an interesting task in some environments. Theoretically, shall statements are individually verifiable and can be clearly written, at least in the microsense. Most of the time, shall statements are poorly organized, ambiguously stated, and difficult to implement or test. They are often redundant and overlapping, yet designers often find large gaps when basic modules are built.
Scenarios tend to focus on current process and change based on current process.
This makes it harder to think outside of the box. Finally, the purely bottom-up actor and responsibility approach raises completeness questions and a concern that generated use cases might reflect individuals' requirements ahead of organizational mission needs. Since each approach has both benefits and limitations, the question of where to start becomes one of basic expediency. Start with whichever entry point offers the most initial return in information. You can begin with what is most comfortable for the organization under analysis or for the team doing the analysis.