Why It's Usually Safe

• Electricity doesn't really care where it is routed and what types of devices it powers in your house, as long as the wiring itself is of the proper gauge and the circuit load limits aren't exceeded. There are many situations where the same main electrical supply line provides basic power to a variety of devices inside a room, such as wall outlets and lighting fixtures. The key to wiring any such devices safely is to make sure the wiring and the circuit is not ever overburden beyond load capacity limits.
  For example, it would be unsafe to try to wire devices (light fixtures, wall outlets, permanent fans, etc) that together would pull more than 2,400 watts of power on a single 15 amp/ No. 14 gauge electrical line designed to carry only 1,800 watts. The result would be a blown circuit, or worse, melted wires and an electrical fire. Safety demands that such a load be carried by a 20 amp/No. 12 gauge line or that such a heavy load be divided between two circuits.
  If the total number of devices on a single circuit is well within 1,800 watts (such as a single 240 watt ceiling fixture) it may be safe to wire one outlet into the circuit---so long as the outlet isn't overburdened later by plugging devices into this outlet that would draw more than 1,500 watts. A microwave oven can pull 1,600 or more watts, which, when added to the existing 240 watt ceiling light, would push the wiring load limits for a 15 amp/ 1,800 watt circuit.
  
  

Why It May Not Be Practical

• While it's possible under some circumstances to wire an outlet into a light fixture circuit, it may not be practical. Most light fixtures are, after all, wired to be turned on and off at a wall switch. If you don't wire the outlet so it functions independently of the light fixture switch, the outlet will be turned on and off from the wall switch as well. This can be inconvenient, and in some situations, even dangerous. You can usually wire around this problem but this technique requires special wiring procedures. Consult a good residential DIY electrical wiring project book for help on this wiring approach.