I get asked to do these things occasionally too, but basically promise nothing, for the very reason you stated.
It does bring back an interesting memory, though, of a friend of mine who got married in September 2009, I think just before 'everybody' had a way to post their snaps all over the internet within minutes. They had hired pro photographers to shoot the wedding. I remember two people moving around with two DSLRs each, snapping away, often in very rapid sequence of some compositions.
I brought a Mamiya 645 with a 300mm lens on it and Delta 3200 film. I shot 30 frames while the pros shot (i'm not joking) 3,600 frames. I had a nice 11x14 print ready and framed for the couple next day, while the wedding photographers took weeks to have anything to present.
So, whenever I get asked to 'attend with a camera' I tell them I charge nothing, and I will accept no direction. Then I process the film as soon as I get home, dry it, and slap it on the scanner. I pick a few good frames, and make decent web size scans of them, and email to those who asked me. And then I make a print or two. By moving quickly (for film) I can keep their interest alive, and have a print in their hands very rapidly. I know what size I'm going to print so I have pre-cut window-mats ready to use, and usually a frame too if they're willing to reimburse me for it. It works well, and usually people keep my print on the wall and at least walk past it every day. I do confess I take some satisfaction in that.