To address your two points, it is bad form for a person driving to move too quickly. If you are not explaining your thoughts and are moving so fast that your navigator can’t follow along you are no longer pair programming and have lost all benefit.
Second, social stimulation can be an issue. But it is important to take lots of breaks, and to have time away from pairing to think and work on your own.
That would make me uncomfortable as the programmer, since I'd want to code at my own pace.
That said, I imagine I'd necessarily slow down if I needed to explain the code, to help pin down a bug or get feedback, for example. So I guess pair-programming would be useful sometimes. I wouldn't like it if I had to do it all the time, though.
With pair programming though, you're not coding at your own pace; you're solving a problem with a colleague and occasionally writing a bit of code. It should be mostly talk - and if there's a bit of grind work coming up, that's when you part ways.
> I wouldn't like it if I had to do it all the time, though.
Few people do, it costs a lot of energy to do social stuff - unless you're extroverted and energized by the social aspect of it.
> That would make me uncomfortable as the programmer, since I'd want to code at my own pace.
OK, but then that's not pair programming. It's you programming with someone watching.