Multitasking comes up a lot in discussions of job qualifications as being a positive quality ("must be a good multitasker"). It also shows up for its deleterious effects on projects by forcing all tasks to be late. And it has been in the news recently with several studies saying it creates problems (NY Times article). So, which is it? Is it good, or is it bad?
Let's clarify. There's the kind of multi-tasking that has you on the phone and responding to email. Or walking and chewing gum at the same time. I'm not so worried about these things. (I am worried about driving, holding the dog and talking on the phone at the same time -- as I saw a neighbor doing the other day.) This kind of multitasking is of the divided-attention variety: essentially nothing gets done well.
Another variety of multitasking is jack-of-all-trades people who work on many different types of activities. This is not a problem in an of itself.
The kind of multitasking that is worrisome is that which creates task fragmentation (a phrase from Tom DeMarco). This comes about when you are a working on a task in a series of tasks (on a project): the next task cannot get started until you complete the current one. This might be for your own work, where you get to manage the damage by staying up late or working over the weekend. Or it might be in combination with other people, where a delay on one task gets pushed down the entire sequence of activities and people. Task fragmentation has you working on five activities all at once, frequently because five different project managers are screaming to get their activity completed.
Working on one activity while another one "bakes" is not this kind of task fragmentation. Putting off pulling the bread out of the oven because you are working on something else is. At least that has an obvious bad effect in the burnt bread.