Has anyone looked at the value earned from crediting rentals to the car companies' own programs (as jackal discussed) compared to airline/hotel miles? I am especially interested in comparison's with Hertz's program.

Since the car rental program (at least for Hertz) is dollar based, and the mileage is rental or day based, then there's some kind of balance between the price for the rental and which way is the most efficient. For example, if you end up with a $10 two-day rental after a coupon, it's probably better to get WN credits or miles/points in some airline or hotel program rather than just getting 10 ZE1 points with Hertz. On the other hand, if that's a $100 rental, then you'd get the same miles/points for air or hotel, but you'll get 100 ZE1 points for the Hertz.

In general, I just assign all my Hertz rentals to ZE1 points, though with some of the promotional codes and short, cheap rentals then it's probably better to assign those to another program. For my situation, none of the alternatives are worth bothering getting points instead of the Hertz points. For certain non-US based airline carriers it might be worth it, but I'd want either HHonors or *wood points which aren't an option through Hertz.

Though if you're never going to get enough points with Hertz to redeem for anything, there's no reason to start collecting them in the first place.