The problem as I see it is that if you have someone working by themselves, then you only have to worry if that person is a good fit for that problem. If you ha

The problem as I see it is that if you have someone working by themselves, then you only have to worry if that person is a good fit for that problem. If you have two people working with each other, then you have to worry if each person is a fit for the problem ... and if they're a fit for each other. Or you have to worry if they are each partially a fit for that problem such that the two of them count as being fit for the problem (and they still have to be a fit for each other). And you also have to keep an eye on overall productivity. You're billing at 2x the rate, so you had better be seeing some noticeable benefit to justify it. I've had good experiences and bad experiences pairing. I suspect that people who like it a lot already have a bunch of experience solo developing and are generally able to get along with a large number of people. Also, they're probably solving easy problems with an overabundance of budget.