I agreed with you al the way up until that very last bit. He's just some guy ...who McCain thrust into the lime light. No one had heard of, or cared about, this guy until the debate.
I think Joe took advantage of an unexpected face-to-face moment with Obama to make a "Gotcha!" move, the details of which were largely fabricated. McCain, without looking into the matter any, in turn took advantage of it to make a point.
The thing is, McCain's point does not rest on Joe's truthfulness. If "Joe the Plumber" had been entirely hypothetical, his argument would have been just as valid, or invalid as the case may be. I think McCain is largely faultless in this, save for making an appeal to emotion over real-world facts and scenarios.
What I object to is making this opportunistic Joe the Plumber into a celebrity, which he will inevitably become on the talk radio circuit.