The point of insurance for certain things is debatable as insurers look for ways not to pay, but pet insurance imho is vital. If you weigh up the cost per week, then think of what you spend a similar amount on (cigs, drink, newspapers, sweets, whatever) it's just a no brainer. You also can't wait until the animal's poorly to think about insurance, therefore it's important to choose the right insurer when they are young and well as once they are older and/or you've put in a claim it's hard to move companies.
Last month we resolved a long running dispute with our ex vet (chronic overcharging, incorrect accounts and nastiness) and we found that his employee/nurse had forgotten to put in a substantial claim in July 09 (£350 odd). She says she posted it but it was never received. We were refusing to pay unsurprisingly as it should have been covered. In the end, the insurance co paid out despite it being months after the claim and we have no pets insured with them any more, both of our boys having passed away. I was hugely impressed and if I ever do get another cat or dog, I'll be going back to them for sure. Over the last years of Swampy's life they paid the vet thousands of £s. We had a 35% excess, but that was because the cats only arrived back in the UK when elderly, so they were an insurance risk. The premium was only £12 a month each as well, even when they were 18 and 15 years old.
I could not have cared for them when they got ill if I'd not had insurance. That would have broken my heart.