Context switching between projects takes time and is a cost to the organization. The more projects involved and the more complex the projects, the higher the cost. By focusing on one thing at a time for a longer time period, individuals can work more efficiently.
So is there a time when multi-tasking is ok? The answer is Yes. Harvard Business Review had a recent article on this topic and they sited a couple instances when multi-tasking could help you.
One idea that struck a chord with me was to multi-task when you are stuck. If you're trying to solve a problem and can't make any progress, step away, go do something else, and come back with some fresh ideas. I do this a lot with work...if I feel like I'm not getting anywhere, I'll go get a cup of coffee, talk to a colleague, or even surf the internet for a few minutes. When I get back to the original task, I've come up with an idea that gets me past my roadblock. Next time you're stuck, try it.