I think, having a prose spec that leaves things unspecified may not be particularly useful in Arc's case; unspecified behaviour in a prose spec is difficult to automatically test for.
Generally, the benefit of a prose, incomplete spec, to program writers, would be compatibility to an abundance of implementations (in either space or time). It helps program writers approach any number of implementers, without actually meeting any particular one. But to make this happen both must read and follow the spec. This is hard on the program writer in particular; he can't automatically test for a spec in the prose, he must read the spec and test his program with an abundance of implementations.
Will Arc's target audience follow a prose spec? Will anyone?