I see what you are saying (that it can be annoying and as an introvert it interrupts your thoughts). I can relate to that, and I have felt that way before. I felt there is no point talking to a stranger (that it is a waste of time because I will never see them again).
Then I realized for me that was too much of a self-centered view point, and it was unkind. And that if I wanted to be treated with kindness and respect, that I needed to give it out first. Some people are lonely, some overstressed (who today is not under stress these days), some have no one to talk to, so are just friendly, some just extroverts who think out loud.
So I concluded that to lend a listening ear and contribute a warm smile and friendly banter, took very little effort on my part, but that it made the other person happy, so why not I said.
In fact just yesterday I was in the doctors office waiting room, and a little one year old girl was crying. So I smiled and said hi to the girl and her mom. Then her mom starts talking to me telling me that this is her foster daughter (her nephew's child) and how her nephew and baby mama are druggies and low lives.
Anyway it took little effort and energy for me to engage in this conversation even though she was a complete stranger. Why would I be rude, what would that solve? So I find it better to show kindnesses (if possible). Plus it actually helps me to relate better to the people I actually care about. I call it practicing my people skills (not my strong suit).
So I think what she got out of it was and opportunity to to vent, to express herself, and what I got out of it was an appreciation of how people live different lives but we all deep down have the same basic needs, to be heard and understood. So I have no problem being polite - but even I have my limits too. For your bus example if it was just a passing conversation it would be no problem, but if I felt like the person was crazy or too creepy, then I would disengage and avoid.