Hi - It's a good question and one that a lot of people ponder; ourselves included. I suggest you ask your insurance agent that you have your auto policy with; we have done that and he said in most cases a well-insured policy holder in the U.S. doesn't need the extra coverage via the car rental company. For peace of mind, ask your agent about your specific policy and to provide scenarios of what would/would not be covered. Hope this helps!
By chance are you staying at any HGVC resorts in Scotland?
We are thinking of visiting Scotland for the first time ... We did spend almost 2 weeks in Ireland in 2010; lovely.