I have a few thoughts I'll make as non-partisan as I can. (Which still isn't very non-partisan. I'm not gonna lie. I don't really like Kerry. I don't much like Bush either. Since I didn't live in a swing state in 2004, I didn't feel obligated to vote for either of them. God bless write-in slots.)
1. If Kerry hadn't said anything noteworthy, what would've dominated the news cycle instead? Probably George Allen's goons roughing up a Democrat here in Charlottesville. The Allen/Webb campaign has already drawn intense national interest for its competitiveness and drama (I mean drama in the lame junior-highish sense), and Kerry blew a great news story for Democrats by shifting the attention to himself, with only a week to go before the election, no less.
2. There are those Kerry defenders who think he doesn't need to apologize for what he said. (What's odd is that his defenders don't agree on whether he was "right" in the sense that he was talking about Bush getting us stuck in Iraq because he didn't study hard enough in school [notwithstanding Kerry's similarly lackluster Yale GPA--let's just settle this right now: you and I, dear reader, are smarter than BOTH these bozos], or right about what it sounded like he was saying about the troops in Iraq. If you can't agree on what he was talking about, isn't that in itself indicative of a problem?) In this case, when Kerry's reputation isn't on the line but his party's is, it doesn't matter whether he was right. Kerry needed to apologize as soon as he realized he'd caused a kerfuffle and get out of the news as soon as possible, for the sake of his party. No, he's not running for office, but he's still the public face of the Democratic party, as their most recent presidential nominee. I doubt nearly as many people know who the DNC chair is, or who the House minority leader is, as know Kerry was the last Democrat to run for president. I don't mean that he actually is representative of mainstream Democratic opinion, just that people are likely to perceive him that way. What he says matters, not just for Kerry, but for all Democrats, and they have a lot at stake this year. (I doubt that ultimately this incident will make much difference by itself, but it compounds the public image issues that Democrats as well as Republicans have. I can't think of an election year in which discussion of actual, you know, policy was more important, and in which less of it has taken place.)
3. Kerry's just a bad politician. He would be much more savvy if he'd had his belligerant response to the Swift-Boating and his meek response to the controversy surrounding his recent comments. I am operating on the assumption that he was foolish enough to say something unintentionally that would alienate a lot of people, but not so foolish as to intend to say what came out of his mouth. I don't think he can be not foolish, though. "Not foolish" entails not giving your political enemies their material in the first place, and then spending two days making the situation even worse. Kerry has decades of political experience and ran for the highest office in the country. He should know by now how the game works.
5 months ago