"Things are the way they are because they got that way." - Gerald Weinberg
XP, like everything else, contains traces of its history - traces that, to an outsider, seem arbitrary. The same is true of functional programming (FP). The histories of XP and FP have had little influence on each other. As a fan of both, I think that's a problem.
Possibly it's more of a problem for FP: I see XP insights being painfully and expensively rediscovered.
It's also a problem for XP. When Agile became popular, XP was eclipsed because (1) it requires a lot of discipline, and (2) it focuses on "that point in the proceedings [where] someone competent has to write some damn code" [told to me by a speaker at this conference who, at the time, wanted to remain anonymous]". As Agile became a management fad, that made XP unappealing.
FP is (arguably) at the point object-oriented programming was during XP's heydey: people with money to spend on programmers are willing to risk it on programmers programming in weird ways. IF XP can