Frankly, that's a terrible idea. It wasn't a good idea Back In The Day, and now in a post-Panda, post-Penguin world it's even worse advice, IMO.
Look, it's time-consuming enough maintaining and optimizing one website. And unless those other domains are filled with quality content and have at least a few high-quality links pointing to them, they're not going to do one iota of good for the main site. The "feeder" sites can only give as good as what they've got. Without thick content and strong links, they don't have much to pass along.
A site owner would be much better served to put that good content directly on the main site, and work to get those high-quality links pointing directly to the main site. There's no benefit to be gained by routing anything through a bunch of secondary sites.
Unless you want to spend all your time working on your websites (and none of it working on your actual business), do what Jill suggests.