To me it seems almost like a bug in the way it's carried through.
It does seem almost normal that Downtownies wouldn't be walking down the sidewalks in the main neighborhood. In fact, it seems a little bizarre when I've played a family that lives on a Downtown lot and regular Townies come walking by.
What is really flawed, though, is if your sim lives in the main neighborhood and is friends with Downtownies and throw parties, those Downtownies won't come to their parties. They also won't go to parties thrown on University lots, although they will visit when invited.
I had one sim last night in college and he threw a party and invited two of his sisters and his girlfriend which is a Downtownie (they're in love--so relationship isn't the problem). I got the message about missing some of your party guests. Low and behold, both his sisters, who live on a Downtown lot (well, it doesn't LOOK like any Downtown I've ever seen
) came with no problem, but Belle was a no-show.