If the wires come disconnected you could have a number of things happen.
1) You'll almost always loose power to the light.
2) You'll probably get some sparking as the wire disconnects. (One of the reasons for an electrical box)
3) The fixture could become electrified if it's not properly grounded. If properly grounded, this short should trip the breaker.
4) You could get a short between the hot and neutral, which should also trip the breaker.
5) With a bad breaker, or undersized wire, a short could cause the wires to overheat and potentially start a fire.
So as you see it ranges from an annoyance to really bad. How are you connecting the wires? If you're properly using wirenuts, you shouldn't be able to pull any of the wires out of a wirenut with a reasonable amount of force.