With regards to the part of your post I have highlighted in bold, I would tend to disagree with.
In my experience, asking someone to sign a model release would make the interaction an entirely 'clunky' exchange.
Sometimes, people are 'suspicious' when asking to make their portrait, they wonder what I am going to do with the image afterwards. On the one hand for me to say its for my portfolio, my people photography skills, as a bit of social commentary only. For then on the other hand ask them to sign a model release makes the interaction 'official'.
Perhaps I am projecting my thoughts/feelings onto this but I just think I would be ultimately suspicious and reluctant to add my signature/permission to some random photographer on the street.
All of the above is my personal opinion, feel free to disregard.