[A]lmost all presidential challengers seem flawed in the early stages. It's the process of winning the party's nomination that gives the candidates stature and unites the party around them.Exactly right. See also a good follow-up from James Joyner.
I'll add a couple of related thoughts, both of which have to do with separating the nomination contest from the general election campaign.
The first is that the quality of the field matters only to each candidate's chances of winning the nomination. Barack Obama will not face the GOP field in 2012; he'll face the nominee. It didn't matter in the fall of 2008 that John Edwards had turned out to be a disaster, and it didn't matter in fall 2000 that Steve Forbes wasn't ready for prime time (let along Gary Bauer and Alan Keyes), just as it didn't matter in 1992 that Paul Tsongas was a lying weasel and the rest of the Democratic field wasn't much better. By fall, the candidates who mattered were Barack Obama, George W. Bush, and Bill Clinton.
So it does make sense to talk about quality of the field when assessing, say, Rick Perry's chances of winning the nomination; he has a good chance because there's no heavyweight running, and some of the other candidates have serious flaws. But that's a nomination discussion, not a general election discussion.
The second point is that candidate weaknesses in the primary season are not necessarily weaknesses in November, and vice versa. So any pro-choice candidate is incredibly weak in the Republican nomination contest, because pro-life groups will veto such a candidate, even though in the essentially impossible event that such a candidate was nominated, he or she might be strong in November. At a more plausible level, we can talk about Mitt Romney's weaknesses in the caucuses and primaries, such as his less-than-fully-conservative past and the possibility that some Christian conservatives might be reluctant to vote for him on religious grounds. But if he's the nominee, no one concerned about abortion on that side would prefer Obama's fully pro-choice position to Romney's perhaps insincere, perhaps surface-deep pro-life position. And while it's vaguely possible that a handful of voters are so anti-LDS that they would prefer that Obama is reelected, it isn't going to be a large group.