Short answer, none. Rental car companies really aren't that paranoid about your driving habits. Plus, there's a lot of labor involved in installing and removing those things.
For a while, Hertz was experimenting with a GPS-enabled system whose main benefit was to provide real-time fuel quantity readings so they could immediately charge you for gas upon return, but they abandoned those.
Most cars already come equipped with XM Radio/Neverlost/etc so installing a separate GPS tracker would be redundant in the event they needed to locate the car.
Enterprise, occasionally, will have a GPS plugged into the OBD port on their cargo vans... why I don't know. I usually unplug it and toss it in the glove box. Only place in the past couple of years I've seen one.