Although I agree with most of the response so far about pair programming being a good thing, I will play devil's advocate and argue that it doesn't always make sense.
When you pair you don't get a programmer that's got twice the brains. You get a programmer that's the union of both of your brains. So basically any time that I screw up and my partner catches or finds some way better, it's a plus. However, any time that I write the correct code on my own is a waste of money since my partner wasn't needed.
Basically, you need to evaluate the code you're working on. Simple tasks are usually not worth the money to pay someone to sit over shoulder and make sure you wrote your for loop correctly. However, at some threshold, the tasks are complicated enough to make the roi for pair programming justifiable.
