[rootsofallevil]

it would be more accurate to say:

Would I bet $20/day that something will happen to this car that would make the rental car company want to be reimbursed for?

Depending on rental car company the limits of scratches, dents etc .. can be very low.

[glandium]

And those scratches, dents, or even more could be entirely not your fault. Heck, they could even happen when you're not in the vehicle.

So effectively it's a bet against you, other people and more generally the world.