When I read anti-Chabad articles, I'm reminded of the Woody Allen joke about the man who tells the psychiatrist, "My brother thinks he's a chicken." The psychiatrist responds, "Bring him in and I'm sure I can cure him." To which the man responds, "I would, but we need the eggs."
1. Does Judaism include the possibility of a dead Messiah? Looking at Chabad's sources (Gemara Perek Chelek, Abarbanel, S'de Chemed), I'd have to say yes, there is such a possibility, although it is extremely marginal within Judaism. (This proves that, thanks to the computer, it is now possible to find almost anything in Jewish sources)
2. Could such a view be harmful to Judaism in the long run? Yes, because it opens the door for Christian missionaries to sell their favorite candidate for dead Messiah to uninformed Jews.
3. In the meanwhile, Chabad is a tremendously vital force in promoting and preserving Judaism, e.g., Chabad houses in the furthest corners of the earth. And, let's face it, such an operation could only be caried out by people who believe they serve in the army of a Rebbe who is the leading candidate to be Mashiach. Otherwise, most of them would pack up and move back to the comforts of Crown Heights.
So, if there were another, non-Messianic Jewish group that was willing to step up to the plate and do what Chabad does for Judaism - I'd endorse them wholeheartedly, and send Chabad to the shrink for treatment. But, there isn't such an alternative, and "we need the eggs", we need what Chabad does for Jews around the world.