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.
Most of the companies I know ofthat advertise this do it because their regular rates are so high it the overtime doesnt even factor in. At my company, we don't charge much extra for weekends because the overhead expenses are 100% covered M-F. Any additional work is pure profit less labor (basically).
Tiger - the old saying, "If it's too good to be true" applys here. There is no such thing as "free." Anyone who tells you that they will work OT hours for the regular rate (unless it's the owner) means the regular rate is already high enough to cover the guys OT rate or more often than not, (as in the case of Mr Electric, or Metro Tech HVAC) they bill on a flat rate. That rate is adjusted to cover OT if necessary. They just tell the customer there's no OT charge.
BTW Tiger - We charge OT after 8hrs, for ALL emergency service calls regardless of time of day, and DT after 8 hrs of regular OT. In 25 years we have hade maybe 1 or 2 coplaints (also, 4hr min for after hr or W/E calls)