Maybe this is getting too far afield, but I would say that "Purpose" is not only a useful, but an essential heuristic in science when it's being practiced by a kind of entity (like human beings) who are hard-wired to think in terms of purposeful action. Making the first question "What is this for?" brings to bear the full power of uncounted generations of field-tested behaviors, rules of thumb, and search strategies.

It is awfully important, though, not to make it the last question. I guess that's where I'd say yes, a "full-blown concept of purpose" in the sense of an unexplained explanation, is unscientific.