1. A passive radiator has the amazing ability to tune a box to almost any frequency while occupying virtually no space at all. Unlike a port, which could displace as much as 0.5 cubic feet of air space within a box, a PR will displace virtually nothing, because it is just a speaker without a motor.
2. Once you move beyond the design phase, a passive radiator is very simple. A ported box often requires extensive box design and more difficulty in construction, because large ports must be "wrapped" around the inside of the enclosure to fit. A passive radiator on the other hand can be modified with the proper weight, and then installation is as simple as cutting a second hole in the box and mounting it as you would a normal speaker.
3. A passive radiator will give you all the great benefits of porting your box in situations where a ported box is simply impractical. These tend to be situations where a powerful woofer is being used in a small enclosure, a great deal of port area is necessary to accommodate that woofer's large displacement, and you want to tune that box low! It is not uncommon these days to run a 15" woofer with 4 liters of air displacement in a 3 cubic foot enclosure with 40 square inches of port tuned to 25 Hz. That port would have to be roughly 50" long, would greatly complicate the box design and completely waste over a cubic foot of air space in the process. A pair of 15" passive radiators, each loaded with 975 grams of mass would accomplish the exact same thing, would be very simple to implement and would displace zero air volume. Not a bad deal.
http://http://momentum.soundillusions.net/august02/tech2.html
