But people aren't single core computers. You're not going to tell me that all you ever do is work on one program. You go play squash in the evening, you have a barbeque in the weekend. The point isn't that, during your work day, you switch between 'job' and 'side project' 5 times; the point is that you work on your job in the daytime, and then at 10pm you hack a bit on that one algorithm you didn't quite get right, or on that lost sunday afternoon while you're waiting for your in-laws whose flight was delayed for 3 hours you experiment with that unit testing framework for language xyz you're been looking at. That's not multi-tasking.