I seldom ask people to take their photo because I prefer to have more anonymous and evocative pictures. However, the times I've done it, people have always said yes.
For my own experience, I find the human connection aspect much more interesting than the photo-viewer connection that you mention. For example, just this week, I had a touching experience as I was waiting for the train. There was a group of rowdy teens and their shadows were just beautiful. I asked them if it was ok to take a picture of their backs and shadows and they said yes. I found it very endearing that they just stopped talking and froze so that I could take the picture. Just a few seconds of interaction, but how kind and caring of them to do so! I still haven't developed the negative, but those are the pictures I feel most comfortable with: where there's a figure, but not an identifiable person. And most importantly, there's also a sense that we made the picture together.
I had another experience that brings conflicting feelings, because I love the end result, but the person is identifiable...so now I have a stranger's portrait...and I find that just weird (it's a photo of a vegetable seller at a market; he asked me to take his picture). To deal with this, I made a print and went back to give it to him. I haven't found him yet, but making a print for him gives a sense of purpose to the experience. In terms of the photograph itself, I like the end result, but have ask for his permission to share it. That's a big issue I find with this: having to ask for permission to show it. It makes me feel like the photograph doesn't fully belong to me if the person is identifiable. I just don't feel comfortable posting it without the subject's consent.
Beyond my neurosis, at the end of the day, I agree with you that asking strangers brings positive experiences overall
