> In contrast, imperative languages with functional subsets do not generally have any well-defined barrier between the purely functional and imperative worlds.

I don't see why this would be a positive thing. You've already lost the theoretical advantage of functional languages, no matter how much of "barrier" you put up between imperative and functional code, and for practical code, you'd be much better off with less of a barrier, because in practice, that barrier will just get in the way of actually solving your problem.