What's on the circuit
Usually lighting circuits are protected by a 15 Ampere breaker. This means that if the total circuit current draw is greater than 15 Amperes, the breaker will trip and open the circuit. It's suggested to only load a circuit to 80% capacity, so the total current draw should only be ~12 Amperes (1440 Watts). The first step in determining if you should add a new receptacle to a circuit, is to determine what's already there and how much power it's sucking. There are many list available online that list the typical wattage of various appliances, that can be used to estimate how much power you're currently using.
Turn off the breaker for the circuit at the main service panel, and make a list of all the items that lost power. Add up the wattage of all the items, and determine if your circuit can handle the load you plan to add.
Electronics and motors don't mix
When motors start (vacuum, circular saw, drill, etc), they draw a large amount of power for a short amount of time. If these items are connected to a circuit that also supplies lights, you may see the lights dim briefly when the motors start. This may or may not be a problem for lights, but you may find that sensitive electronics (TV, computer, etc) don't like it so much.
Blow a fuse, wander through the dark
It's often recommended to keep light circuits separate from other circuits. This is done for the simple fact that if you trip the breaker, it's nice to not be standing in the middle of a pitch black room because of it.
You get your project all ready to be cut, you line the saw up, pull the trigger, and click. You're standing in the dark.
Circular saws tend to be rated 12-15 Amp, which means they'll push most circuits to the limit all by themselves (15A * 120V = 1800W). This is why it's a good idea to not mix lighting and motor loads.
Is the wiring already in place
In some cases, only 2 wires exist at the switch (excluding the equipment grounding conductor). There will be a wire carrying electricity to the switch, and another that carries it to the light when the switch is on (closed). In a situation like this, you'll have to run more wire to connect a receptacle.
In other cases, the power for the circuit passes through the switch box. This is the perfect situation, because you can use the power at the receptacle before interrupting it with the switch. In this situation, you should see 4 wires (2 cables) in the box (not including the equipment grounding conductors, which may or may not be present).