I had two on yesterday's Meriden 100 - presumably not the same ones as De Sisti, unless they were rather lost.
One (not, I think, a natural audaxer) who had lost his group on the main climb and was now navigationless, who asked if he could ride with me to the next control. I agreed, with a disclaimer about pace, and we had a nice chat on the flat mostly tailwind section. The problem with this is you start to feel responsible for the other person, which makes the whole thing much less relaxed. (This wasn't helped by us not seeing another rider for ages, which left me wondering if I'd gone off-route.)
And later on, another who lurked 10-20m behind me for miles (except where I was considerably faster on descents) without ever catching up and saying hello. I eventually stopped for a knee break, and he overtook, asking if I was okay as he passed. So not impolite, just really disconcerting.
I suppose the latter is a bit like when I catch up with the back of a group, but decide to stay there rather than play leapfrog (I tend to do this if it's a narrow lane or the group obviously lack awareness/discipline with regard to overtaking), or because I don't have enough speed reserve to get past. It's much less creepy with a group, though.
Related: There were a small group of riders who I kept meeting, who seemed to think that the way to let a faster cyclist pass while riding two abreast is for the outer rider to move to the far right of the road, so the overtaker can pass through the middle. This confused me immensely.
