I think you may have missed the whole S2X vs UASF battle. There are multiple important reasons for non-mining full nodes to be validating the whole chain and not just their own transactions.
Having many independent observers seeing the entire chain and detecting problems or attacks against it, facilitates coming to consensus out-of-band about what's happening and what to do about it. It's the ultimate check-and-balance.
Having many independent observers seeing the entire chain and detecting problems or attacks against it, facilitates coming to consensus out-of-band about what's happening and what to do about it. It's the ultimate check-and-balance.