The reasons I don't buy comp ins but only stay with liability ins with higher amount are - I believe the chance of my fault is slim, and for minor loss I wouldn't get paid from my own insurnace because of deduction amount if I did purchase the full coverage. This is similar to, although not the same, the case when you buy a LCD TV or a laptop computer, did you or how many savy people buy extended warranties? When you purchase your home insurance, did you buy earth quake coverage, too?
On the other hand, I buy liability coverage, because (1) law requires it; (2) higher amount - to protect me from malacious lawsuits against me in case I was involved in an accident that I were the one responsible. I could handle the loss of fixing my car, I had no control on other party's shrewed lawyers what he was gonna do. Higher mount is to protect me.
The cost of liability is about $700/y vs nearly $2000/y for comp coverage in my case. Different people have different opinion, I am happy with what I set up so far.