From a safety point of view, it is very difficult to create a situation where a light fixture can draw more current than is safe in 16 gauge cord, but still less than enough to trip a 20 amp breaker. Typically you're either going to be drawing around the normal amount for fixture, or else somewhere around a dead short.
The code appears to take this into account in the cord and the tap rules.
This is presumably part of why receptacle adaptors that screw into an Edison-base socket are frowned upon. And also why outlet strips do require built-in circuit breakers--it is well within the realm of possibility that someone can plug enough things into the strip to overload the cord, but not the circuit.
