I've always liked pairing because it ensures that you're both there to get the thing done, and generally I've found that it keeps both of us from goofing off (until the thing is done, then it's time to play). While the theoretical throughput would be 50% (two people doing one thing), it ends up being greater than if we had both tried to work on different things, since the one thing we're doing comes out better since it's getting reviewed while written + any problems which come up can be solved much much faster. Plus, both people always learn at least one little thing, typically more.

Pairing is just great. Works well for sysadmin stuff too, even if I'm mostly leading that and just bringing someone else along for the ride.