A PLA is reasonably space-efficient if you need gates with a fixed set of inputs and outputs. Even the 8008 used one for instruction decoding. In addition to the microcode ROM, the 8086 has several PLAs of various sizes. For instance, one PLA converts the ALU operation into the necessary ALU control lines.
A key distinction is that in a ROM, only one row is active at a time. But in a PLA, multiple rows can be active simultaneously.(This is from "The Architecture of Microprocessors".)
