Thank you all for being my collective conscience! I think it was the right decision to go, and the various arguments gave me lots of food for thought. That said, Watson's talk was even more pointless than I had suspected it would be.
First of all the organizers did something very stupid, namely scheduling the talk for a tiny little lecture theatre which was in no way big enough for such a famous speaker. It was also hard to find, so I was ten minutes late, by which time the place was absolutely crammed like a pre-Hillsborough football stadium and I found myself at the back of the crowd in the doorway. Obviously it was impossible to see or hear from there, and it was also boiling hot, and I was very tempted to give up, but in the end patience paid off and enough other people gave up first that I ended up just inside the doorway.
So I heard the lecture standing jammed between sweaty bodies, which may not have been the ideal circumstance. My expectations were not that high; I assumed Watson would follow the pattern of most grand old men and speculate a bit about where biology is going, with a little bit of gossip about his career. But he didn't talk about biology at all, and realized that he is famous enough that anyone who cares knows everything about his career already. Instead he provided some careers advice, most of which was obvious, and a lot of which was more applicable to 1950s America than modern Europe. So even if I had been younger and more inclined to hero-worship, I don't think I would have got a lot out of it. I think the most interesting he said was that it's very important to collaborate, and you'll never get anywhere if your goal is personal glory; you must ask for help if you're stuck, you must talk to others in your field even if they are your rivals, you must seek out people who are more intelligent than yourself and who are experts in related fields.
There was a little bit of "yay, atheism", and a little bit of dismissing RNA biology as just a fad (I think he's wrong there; DNA is important, sure, but it's part of a system, you can't these days get away with only caring about DNA). But he rather lost the thread of what he was saying, overrunning his allotted time by rather a large margin, and degenerating into a rambling diatribe against Franklin by the end. He's apparently moved on from calling her unnatural and unfeminine, to calling her "autistic", which he probably thinks is less offensive. It really did just come across as completely pathetic; the poor woman has been dead for half a century. He didn't appear to notice the irony of complaining (at excessive length) about how Franklin was arrogant and difficult to work with, where in another part of the talk he commended us to work with scientists who have a reputation for being arrogant and to care more ability than niceness. He was also rude about Linus Pauling, so I suppose he isn't only misogynistic.
He made a bunch of random snide, but not actively offensive, remarks about Jews (the whole of biochemistry in the 40s and 50s was massively dominated by Jewish scientists who had fled Germany and Russia, and indeed most of Watson's major scientific influences were Jewish). He was extremely rude to Georg Klein, who was as usual sitting in the front row; I don't know their history enough to know whether it was just friendly teasing or being deliberately offensive.
I didn't bother staying for questions; it was too hot and crowded and Watson had really lost my interest by that point. Oh, he did make one rather cute remark to finish; he said that if they ever made another film about him he would like to be played by Sacha Baron-Cohen.
I cut him slack for being vague and rambly on the grounds that he's eighty. But I don't cut him slack for his sexism for that reason; he's just not old enough to remember a world where it was reasonable to assume that women are naturally incompetent and all the serious work is done by men. Female scientists were still the minority back in the 50s, but hardly unheard of; he himself mentioned that Franklin was one of a couple of dozen women in the chemistry department at King's College London. Even if has been completely unable to adapt to the changes in society in the past fifty years, there's no excuse to make comments about how you should make sure to spend time in conversation with other scientists and not waste too much energy gossiping about politics with your wife. I don't think he even intended that remark to be offensive, unlike some of his comments about "the feminists" who were so meeeeeeeeeeean to him and made a totem of Franklin just because she was a girl even though she wasn't particularly competent (sic). He just unthinkingly assumed that all scientists are men, and women only talk about trivial things. I think for someone to be too old to understand that women are people, he would have to be at least 150, which is to say, there's no excuse any more.
Anyway, the evening was much improved by an invitation to dinner with EBH, which was as usual delightful and full of interesting, intelligent conversation. Just what a Friday night should be, in fact.