2 parts to the thought process. In part 1, you can think that America is not ready to elect a black man, woman, atheist, gay person, non-human, what have you. That's a fairly neutral statement which might or might not be true. In part 2, you say you won't support this supposedly "unelectable" person because they won't be elected. THIS is the problematic part. There are some years when the writing was on the wall that some candidate for president or another wouldn't get elected (Mondale, Duakakis etc.) Would you vote for their opponent because that person would get elected? There are lots of times when people vote for someone who they know won't win because they support their platform or just like them.

Also, I hate to sound so cynical, but because 1 vote or a small group of votes doesn't usually change the outcome of a large election, voting for Obama (or Hillary or Kucinich) even if you think them "unelectable" isn't likely to cause them to win all by itself. So you really have to assess the motivations of someone who is calculating that way.

Now it is true that in "unelectability" in and of itself can make a candidate unattractive (Chuck Pennochio), and that's a legitimate feeling, but that's different. I think what you're talking about is a situation where you genuinely like a candidate, but feel you can't vote for them because of some sort of electability calculus.