This also acts as a public filter
If you small talk with someone and they react positively (or at least don't treat you weird) ... then the person you are small talking to is a decent RECEPTIVE person who is also open to a bit of communication.
If you small talk with someone and they immediately treat you weird, walk away or just don't want to deal with you ... then chances are, it was a good thing you said something to eliminate any negative interaction.
EDIT: changed a word in my phrasing