How many people build their website empires under one single domain name? How many people practice this?
It depends entirely on what you're wanting to achieve. A network of blogs, for example, would best be served with each blog having its own identity (domain, design, etc.) A bunch of tools related around the same topic would best be served under one domain. About.com, ebay, etc have all done a great job of using subdomains to serve traffic / for SEO purposes. Weblogs Inc and numerous other blog networks have done a good job with the flipside.
jdg makes a good point - it all depends on your industry and target audience. If the audience of one of your sites doesn't want to look at the info on the other subdomains, then this strategy wont work as well.
It's quite dangerous to do this. If your main domain or subdomain gets penalized by a search engine, you will most likely be penalized for all.