Not at all. Even if there is nothing to it theoretically, I think it's valid for practical purposes -- just as when in engineering one refers to a "centrifugal force" for convenience.
As I said, if I (changing the example a bit), refer to a Fi-ISFj, the image you get is different, and more precise, than if I just refer to a "ISFj", since you know that the person is little like a Se-ISFj.
But I am wondering (still undecided) whether it's really worth it to develop complicated theories as to how being this or that subtype is going to affect the 6th or 8th function or whatever.
I think that individuals of a given type may get too focused on their 6th or 5th function (for instance), but not necessarily because of a subtype.