I agree with Kevin that the idea that product managers will write cucumber tests is pretty weak. They might well read them and understand them however.
I think, however, that the value of cucumber and its gherkin syntax are, as with many things, not exactly in the place where the designer thought the value would be. The value is in using anything but ruby to describe what you're trying to accomplish with ruby. Every so often I'll write or encounter a test written with the same narrow view with which the programmer wrote the code. Such a test verifies that the code reliably does the wrong thing in the larger sense. Using ruby to test ruby also encourages trivial tests where the implementation detail is verified, not the application logic (see Don't Unit Test Trivial Code )
My science-based (but not scientific) theory is based on how low-level thinking impedes high-level thinking. As an example, this happens when you're driving, thinking about something low-level like adding distances, and get distracted and miss your freeway exit. Cognitively, when you're in the middle of writing Ruby code and thinking about how to write Ruby, it's harder than normal to think "what should the code do". Switching to another language, gherkin or anything else, prompts the programmer to go meta. Going meta means repeatedly re-loading the mental model of how what i'm doing is fitting into a larger system and goals.
This effect is known in a couple different fields:
- Learning math: "Good problem solvers possess metacognitive skill, the ability to monitor and assess their thinking" (ref Support for Learning)
- Corporate strategy: "A strategic thinker has a mental model of the complete end-to-end system of value creation, his or her role within it" (wikipedia Strategic THinking)
So besides just changing to another language to avoid ruby-testing-ruby circularities, Gherkin is designed to make the programer think in terms of wants and fulfilling user expectations. The syntax "Given I am a new user, When I go to the home page, Then I should see the zero-content display" helps the developer pop from what she's trying to do to how she's trying to do it, and back, without losing the big picture.