Reading, "Organizational Patterns of Agile Software Development" it suggests code ownership because "Something that is everybody's responsibility is really no one's responsibility". He does note some concerns related to one owner of the code including, "...tunnel vision, the implied risk of having only a single individual who understands a piece of code in depth, and a breakdown of global knowledge." as well as the bus factor and introduction of bottlenecks.
This disagreement between XP and Coplien's agile software development was noted by Kent Beck in a 1999 interview: "KB: I was talking about it with Cope [Jim Coplien] yesterday; this is our favorite fighting topic. The rule is, if you see a problem with code anywhere in the system, you fix it. So we’re sitting there with our DB40 transactions, and we say, “Y’know, if this export object over here was just structured this way, it would be really easy for us to do our stuff.” If it would clean things up else-where, we just do it."
"JV: So anybody can change anything, anywhere…
KB: Yup, absolutely. If you see it, and you got your partner there, and you’re going to run the tests within a few minutes—so it’s not like you’re going to break something—you just do it. And the system’s going to get better.
Now, I came up with this because I was working in strict individual code ownership shops, and we’d say, “Gee, we keep calling these same three methods in this object. Why don’t we just make a method in the object that calls the three methods for us?” “We don’t own that.” “But the guy’s just across the hall—” “Naah, don’t wanna bother him.”
The pace of evolution in projects that use individual code ownership, in my experience, is glacial compared to what is possible to do, in a controlled way, if everybody takes responsibility for making all the code as good as they can make it."
"JV: Has this been without pitfall?
KB: Yes. It’s just not a problem. You’ve got to have collective ego instead of individual ego. That’s the hardest problem, so that no one comes up and says, “Hey man, you changed my class.” "
See also: Ron Jefferies' Code Ownership and a list different kinds of code ownership.