Can a side gig help your career?

May 17, 2012, 2:14 PM UTC

FORTUNE — Moonlighting has always been part of American work culture, though it’s not a lifestyle many managers have encouraged. According to the Bureau of Labor Statistics, only about 5% of workers officially hold more than one job. Some organizations have policies against extra hours work, both for liability and productivity reasons. There are 168 hours in a week, and time you’re spending at a second job or on your own side business is time you’re not dreaming up new ideas for your employer.

But changes in technology and the way people work are leading some to rethink this idea. Certain kinds of moonlighting may actually help you in your main job, and wise organizations can embrace, rather than squelch, entrepreneurial zeal.

The key insight is that while the term “second job” conjures up an image of commuting to a second site after a long day at the first, these days “I don’t have to move my atoms around,” says Paul Kedrosky, a venture capitalist and senior fellow at the Kauffman Foundation. Platforms like Etsy (where people sell crafts), eBay (EBAY), Zazzle (where people hawk designs) or Quirky (a crowd-testing site for manufactured products) allow people to do creative work from their home computers.

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Bloggers can make money from ads and people with special expertise can teach virtual courses. There’s no real difference from an employer’s perspective between someone sitting in front of a computer until 2 a.m. and a television until 2 a.m. — which managers have never been able to ban. These side businesses, which Kedrosky calls “fractional entrepreneurship,” should be of “no more concern than having hobbies.” Indeed, modern moonlighting often involves things that might once have been hobbies.

By day, Martin Cody is a vice president of sales at a medical software company. By night (and early mornings and weekends), he’s the president of a business called Cellar Angels, which uses a Groupon-type model to get members discounts on wines from small wineries while bundling donations for partner charities. He and his wife also own a retail wine store in Chicago. Because he’s two hours ahead of his partner wineries in California, he can work on Cellar Angels from 5 p.m. to 7 p.m. while preserving the hours of 8 a.m. to 5 p.m. predominantly for his day job. While the combined 70 or 80-hour workweeks are long, he notes that he and his wife don’t have children, so they can work on their side venture during the time that other people are carting kids to soccer practice. And plus, “I don’t watch ‘SportsCenter.’ I don’t watch football games,” he says. He’d rather be thinking about wine.

Some people even find that a second gig offers synergies with the first. Beth Henary Watson is the executive director of the Mineral Wells Area Chamber of Commerce in Texas. She and her husband recently bought a hair salon called All Star Clips in Weatherford, Texas. In her day job, she advises small businesses on problem solving and growing profits. Now that she owns a small business for the first time, her advice has real world experience behind it. One example: Watson knows that most of the All Star Clips clients are male. “Clearly that’s who we should be trying to reach more of. I use this knowledge acquired first-hand to tell our chamber members that they should target, target, target, and ignore, politely, everyone else.”

“It makes a lot of sense for someone who works to help businesses to have owned one before,” she says.

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People who develop social media skills to promote their side businesses can put that knowledge to use in their main gigs. Content knowledge is also useful. Martin Cody has a lot of conversations with his physician clients about wine; Jennifer Teates, a firm manager of a small Annapolis, Maryland law firm, finds that her freelance work as a personal finance writer has given her more confidence during 401k discussions at her main job. “I am more confident in asking questions and offering opinions,” she says.

A stronger social network may be a plus too. “For example, if you’re involved in a nonprofit or community organization, your good works may provide some kind of benefit-by-association for your employer,” says Marci Alboher, author of One Person/Multiple Careers. “A side business can work the same way if it helps you make new connections that can help your main employer.”

But even without these obvious synergies, tolerating or even supporting employees’ side gigs has another upside, notes Chris Guillebeau, author of the recently published The $100 Startup: Reinvent the Way You Make a Living, Do What You Love and Create a New Future. You attract entrepreneurial people with lots of energy. This is one thought behind some organizations giving unlimited vacation time so people can pursue whatever projects they want outside of work. “Think about it — most employees are [at their jobs] out of dependency,” says Guillebeau. Those that have other options — such as side gigs they could do full-time if they wanted — are there out of choice. “Those are going to be the employees you absolutely want,” he says.

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There are limits, to be sure. Working to launch a competing product on the side is a no-no, and blogging about your employer’s flaws is not a good call. And if someone aims to start a really big business, eventually that needs to become the day job. “The key in all instances is to make sure that your side business isn’t so consuming that you neglect your day job, and that it doesn’t present any conflicts of interest,” says Alboher. But if designing jewelry on weekends can make an employee happy (and potentially less pushy about asking for a raise) it’s hard to see why an organization wouldn’t get on board.