I would argue that it's dependent on what you're trying to accomplish. Sure, you learn a ton, but it still comes back to the issue of how well you can perform when you're trying to juggle a side project, a day job or whatever else you're working on.

Personally, I've found that when I do multiple large projects at once, I sacrifice performance on each. There is a tradeoff in what you learn, but I'm not sure it's an equal tradeoff.