My interest in setting up a main sister site has nothing to do with revenue, its about having an umbrella in case my business site ever falls in the SERPs.
I have considered setting up one or two individual sites for specific types of industries, that would be for increasing revenue. I did set up one such site three years ago for a specific niche, only its completely different from my main site and isn't remotely connected by linking, its been at the top of the SERPs for years. But if I set up another industry specific design site, I'd be tempted to link to both sites from any new project I develop within that industry, and maybe that's something Google will notice and disapprove of.
[edited by: DXL at 10:38 pm (utc) on Nov. 29, 2007]