I think the lock feature on the Nectar app is a very good idea. Before they introduced it, I had over £70 worth of points used at a garage in Enfield, not me! Although having bought petrol at a Sainsbury's garage locally, I did wonder if somehow that was the source of my missing Nectar. I did get them reimbursed, their tech department were helpful. Although not everyone who has had that happen to them was so lucky I gather. In fact I started a thread here on GN about stolen Nectar, around the same time as reading an article on the subject, it was becoming a problem for Sainsburys hence the lock feature on their app. Wise to keep points locked until ready to spend them, I imagine there's no comeback since it's implementation, that onus is now on the Nectar card holder.
It's worth having a Nectar card if you happen to be a Sainsbury's shopper, same with The Tesco Club card, many products are discounted.