Self-checkouts are less efficient but certainly are more cost-effective for the store using them. Cashiers are not just "check-out functionaries" but are also valuable as sources of information, recommendations for other products and the uses of the products being purchases, and they go a long way to helping the store's "brand" of being friendly, personable, and approachable for the customers. That "human face" is a huge factor, in other words.
That being said, I think we will see that more and more companies move to automated checkouts to save money. When there's a cheaper and better way to do something, that will usually carry the day. As far as the companies are concerned, I think they will view the cost of a few extra seconds for self-checkout and the cost of having only a skeleton staff to assist those having problems with the checkouts as a viable alternative to paying salaries and benefits for "live" employees.
If we can be replaced by automation in our jobs... we will be.