If two people on a team regularly contribute to a repo, one person likes to work on feature A, and the other feature B, and all of a sudden, the feature B person leaves and never comes back, that will kill velocity. Even if feature A person has been doing code reviews, a code review is much different than being aware of the system and continuing to improve a feature. Now person A has to either read documentation (which is, for the sake of code writing, a 0 velocity), or take larger tours through the code to figure out why a feature is how it is, and what needs to be done.

So the big question for anyone is if they want to focus on velocity/lines per day, or consistently smooth development. You can get both of those things when everyone is doing their own tickets, but that will immediately stop as soon as there is a change in your team.

I don't think pairing is perfect, or mobbing for that matter, but it does give clients a more consistent/smooth experience if they are okay with paying more for quality than speed. It does take the right kind of people and the right kind of project, just like any problem and tool set.