perhaps the use of somebody's name often occurs as a more general signal that they're exasperating you or being slow in some fashion.

That certainly fits with the way I do it. I almost never say people's names to them unless I'm having a hard time getting a simple point through to someone I suspect of missing the point on purpose. Naming no names, obviously. Otherwise, the only time I've really come across it is in salesmen.

Yeah, I was going to mention that it's very commonly done by salesmen and related lifeforms. A bit like the real-life version of a mail-merge.

And they seem to think it makes them sound more like your chum -- look, they're addressing you by name as if they've known you all their life! -- even when your real mates likely never address you by name except at the beginning of speech to get your attention.