The economic theory is pretty simple, you hope the increased business offsets any additional cost of doing business. If you satisfy a customer at 8 p.m. there's a good chance he'll call you at 8 a.m. too. In terms of running the business you have three choices, eat the additional labor cost, increase your "straight time" hourly rate to cover the cost (or at least part of it), or hire someone who wants to work the second shift at straight time.
Today, with so many two earner families and high child care costs, it might not be that hard to find someone who wants work a different shift. And there are some guys who just like the secord (or even third) shift.
A while back I needed a plumber on a Saturday morning. $175 to walk in the door. $100 for OT labor plus a $75 "emergency" fee. When looking in the yellow pages if I had seen an advertisement for "straight" weekend time, I would have probably called that company.