The employee running his own business is going to be somewhat distracted, just like an employee with a family or kids. It's something else that is more important than his job. The thing is, an employees interests don't exactly align with the business owners interests. As a business owner, almost everything I do is focused on growing that business. The closest you can come to that in an employee, really, is a single person who is really into programming. these people will have hobby projects, but that's ok, especially if they can be things kindof related to technologies your business uses. Like the SysAdmin who writes apache modules.

That said, you don't always just fire people when they get married. the question, simply, is 'Is the employee, with all his or her distractions, still worth his or her salary?' - Most of us are unwilling to remain single and completely dedicated to our jobs for our whole life. Right now, for instance, if you want to hire me, you have to put up with my business being my primary focus, otherwise I won't work for you. (but then I'm a contractor, so that's not unexpected.) And just imagine the outcry if you acted against people who valued their family over your business.