I guess it depends on a person. My experience is close to that of 'ryandrake.
I've been coding long enough to notice there are times where the problem is complex and unclear enough that my own thought process will turn into pair programming with myself, literally chatting with myself in a text file; this process has the bandwidth and latency on the same order as talking to another person, so I might just as well do that and get the benefit of an independent perspective.
The above is really more of a design-level discussion. However, there are other times - precisely those times that pair programming is meant for - when the problem is clear enough I can immerse myself in it. Using the slow I/O mode, being deliberate is exactly the opposite of what I need then. By moving alone and focused, keeping my thoughts below the level of words, I can explore the problem space much further, rapidly proposing a solution, feeling it out, proposing another, comparing, deciding on a direction, noticing edge cases and bad design up front and dealing with them, all in a rapid feedback loop with test. Pair programming in this scenario would truly force me to "use the slower I/O parts of your brain", in that exact sense: it's like splitting a highly-optimized in-memory data processing pipeline in two, and making the halves communicate over IPC. With JSON.
As for bus factor, I find the argument bogus anyway. For that to work, pair programming would've to be executed with the same partner or small group of partners, preferably working on the same or related code modules, daily, over the course of weeks at least - otherwise neither them nor I are going to have enough exposure to understand what the other is working on. But it's not how pair programming worked when I've experienced it.
It's a problem with code reviews, too: if your project has depth[0], I won't really understand the whole context of what you're doing, and you won't understand the context of my work, so our reviews of each others' code will quickly degenerate to spotting typos, style violations, and peculiar design choices; neither of us will have time or mental capacity to fully understand the changeset before "+2 LGTM"-ing it away.
--
[0] - I don't know if there's a a better, established term for it. What I mean is depth vs. breadth in the project architecture. Example of depth: you have a main execution orchestrator, you have an external data system that handles integrations with a dozen different data storage systems, then you have math-heavy business logic on data, then you have RPC for integrating with GUI software developed by another team, then you have extensive configuration system, etc. - each of those areas is full of design and coding challenges that don't transfer to any other. Contrast that with an example of breadth: a typical webapp or mobile app, where 80% of the code is just some UI components and a hundred different screens, with very little unique or domain-specific logic. In those projects, developers are like free electrons in metal: they can pick any part of the project at any given moment and be equally productive working on it, because every part is basically the same as every other part. In those projects, I can see both pair programming and code reviews deliver on their promises in full.
