Here is a piece of cynical thought: Extreme Programming works not only because of its inherent traits, but also because of some interesting side effect.
Let's start with two simple questions: how many times have you been interrupted in your work today; be it by your boss, your superior officer, or one of your colleagues? What were you doing when you experienced the interruptions?
Those of you who, most of the time, work solo in your cubicle probably experience the most interruptions. And you were probably staring at the monitor, trying to make sense out of some important piece of code or technical document.
The reason you get interrupted is because other people do not believe that you are doing actual work, regardless of how legitimate your endeavour was. Think about it: when you are staring at the monitor, most likely the font is too small for a bystander to read with ease. If he cannot read with ease, he won't bother with reading and will not be convinced that you are busy. Therefore he assumes that it's ok to bother you.
Now, have you ever noticed how, sometimes, your boss walks towards you, only to realise that you are on the phone with somebody business-related, so she walks away within 5 seconds without bothering you? The reason your boss walks away is because she can hear all kinds of jargon in your conversation, so they assume (rightly) that you are onto something important.
I have come to hypothesise that one gets the fewest useless, unscheduled interruptions if he picks a working methodology that involves lots of talking, thereby letting uninvolved parties hear lots of jargon. When people hear jargon, they suddenly find reason to believe that you are doing actual work. For this reason, Extreme Programming becomes a good candidate. When you practice Extreme Programming, you are inevitably paired with a colleague, sharing one computer. When two people work together at the same computer, they inevitably talk a lot more than if each of them were working solo, and therefore they will let people around them hear a good load of jargon.
There you have it, the side effect that contributes to the effectiveness of Extreme Programming.
Note: this post is entirely meant as a meek piece of I.T. workplace satire. It is not to be taken seriously. Dear Kent Beck, if you happen to be reading this, please don't get mad.