rental car insurance question

We reserved a rental car for 4 days, leaving from Oriente station in Lisbon. (If this is not a good idea for some reason, we could also do the airport -- open to thoughts on that.)

Because it was only 7.50 more/day (so 30 euros total) I paid for the insurance with refundable excess or something like that -- includes liability, CDW, and refundable excess (including glass, tires, etc) with some exclusions I thought were reasonable.

I was thinking about it after ... 30 euros is not much to pay for peace of mind ... but if I didn't pay, I have some coverage (but not liability) through my credit card. It occurred to me that when I rented in New Zealand, liability was included as a part of ALL car rentals (or something like that). Should I pay the extra? Am I just spending money needlessly? Thoughts?

I never buy insurance at home but in case something happens, i might prefer to just walk away...