Another reason that composite systems may result in poor matches to original faces is the principle of "blended memories."

... it is possible that building a composite face creates a second memory; there are now two memories of the face, the original memory and the composite memory, and they compete in any later memory task. ....it is [also] possible that the original memory is blended or averaged with the original memory, yielding a new face that has some characteristics of the original face and some of the composite face.

More possibilities:

It is possible that the original memory of the face is replaced with the composite face, yielding only one face in memory, namely that of the composite. There are also hybrid possibilities that involve more than one of these processes.

I think also that the more the sketch is publicized, the more likely the witness will be to compare his or her memory of the perpetrator to the sketch rather than to the person they saw during the event.

Nonetheless, Wells says that composite sketches may retain some value in criminal investigations. He concludes:

On the one hand, we know that composites usually produce results that poorly resemble the actual face that they were meant to depict (citations omitted) and that composites can apparently damage the memory that the witness has of the original face (current research). On the other hand, we do not know how often composites somehow manage to help crime investigators eliminate potential suspects or narrow the search of possible suspects. Furthermore, in multiple witness crimes it might be possible to use one witness to build a composite and save the other witnesses for any later lineup identification attempts.

Wells cautions that his findings may not be applicable to police sketch artists.

Furthermore, we cannot be certain that the effects we observed here are applicable to sketch artists. Like computer and transparency-based face composite systems, the sketch artist process is somewhat featural based rather than more purely holistic. Unlike computer and transparency-based face composite systems, however, the sketch-artist process does not require the witness to examine isolated facial features and select among them. Hence, we reserve judgment on the sketch artist process until we more fully understand the processes leading leading to the effects that we observed here.

So yes, be on the lookout for the man in the composite sketch. Report him to police if you see him. But don't presume he is the murderer. Whoever committed the atrocious killings of Mr. Lefkow and Mrs. Humphry needs to be apprehended, sooner rather than later, but if police nab the wrong guy, the killer(s) remain at large, able to kill again.