I guess because Sesame Street is such an informal atmosphere, I think the object was to make the humans not just adults, but friends for the kids as well, hence why we all grew to know them on first name basises; there are virtually no parents to the Muppet characters on the show (for the most part) to tell them calling grownups by their first names is impolite, and that they should be addressed as Mr. or Mrs. or Miss, etc. Heck, I remember when I was in a dance class as a kid (yes, I was in a dance class, shut up), the instructor encouraged us to call her by her first name, and all the kids did except me, because my mom told me it was impolite to do so. PBS was different though, because it was more of a working atmosphere, so I was free to call everyone there by their first names.
Since all the kids I know in my life are basically relatives (nieces and nephews mostly), I never hear anyone refer to me as "Mr. Scarbrough", lol. We all know Steve calls me "Joe" instead of Joseph since he has no respect for me, but ever since then, every new person in my life calls me "Joe" too. Frankly, I could care less if a kid calls me by my first or last name - I'll be formal for formal occasions, but I'm not a formal person by nature.
But back to the original topic - I don't recall ever actually hearing Robinson on the show, but I do know that that is Gordon, Susan, and Miles's last name, as it says so in Sesame Street Unpaved (and yes, it also mentions that Robinson was probably in honor of Matt Robinson). I remember when the fire broke out in Hooper's Store, Maria called 911, starting with "Hello, I'd like to report a fire. My name is Maria Rodreguez, I live on Sesame Street, and there's a fire in Hooper's Store". When Gina had her vetenary office, the name plate on her door read "Dr. Gina Jefferson". Also, getting back to SSU, it seems to me that anyone who share their name with their character is basically playing themselves - both Bob and Linda's full names are the same as in real life (McGrath and Bove, respectively), so one can only assume that Alan shares his last name with his character as well.