Perhaps you're one of those super-organized, aim-driven people who likes to start a project and finish it - one of those writers who will dedicate every spare moment to a single novel, until it's finished and polished. I confess I'm not one of those people. Though starting something, working on it and it alone, and finishing it may seem logical and efficient, it just doesn't work for me.
Whichever way you look at it, a novel is a long haul. Though with my 1,000 words a day schedule it's possible to turn out a first draft of a 100K-word novel in 5 months, usually novels take longer, up to several years - don't forget the editing process. And I really need that creative lift that comes from pure, unadulterated first-draft writing - you know, the kind when you don't worry about the turn of a sentence because you're so busy flying on the wings of inspiration.
So, what I usually (almost unconsciously) choose to do is the following: write the first draft of something, start editing/proofreading, and begin a new project at the same time. It does provide a distraction and slows down the editing of the first novel, but I find that if I try to stick to working on just one thing at a time, I'm a lot more likely to encounter writer's block.
Right now I have two such projects on the go: Wild Children, which is my very first sci-fi dystopia, is in the process of being edited. At the same time, I'm writing Stronger Than Blood, which is a Quest of the Messenger spin-off. Each one could perhaps be finished more quickly if I devoted a chunk of several weeks/months to working on it separately. But since I'm only human, and since I need to enjoy writing to keep doing it, I will stick to my way.
I do believe there are definite pros to working on several projects at a time, too. It enables you to step back a bit, get your mind busy with something else, then come back later and view your work with a more objective, critical eye - like staring down from the hilltop. I've had some really good ideas after stepping away from something, doing something else, then going back to the first thing.
But ultimately, again, I'm doing this the way I enjoy most, because writing is my primary creative outlet and I can't allow it to become a tedious chore.