Choice. If you have only one line to your CPE, you have none. If there are multiple lines (ISPs) to chose from - you can select the best one available. As the ISPs have to actually compete for customers (assuming they don't collude to screw everyone - which is a legal problem) and any bad behavior will be punished by losing some market share, their customers will benefit from this.

Also, resilency/failover. If one connection goes down, you have a backup. It may be uncommon, but for some people who rely on Internet connectivity, this is useful.

Single provider just can't know best.

Talking about efficiency... For efficiency's sake it may make some sense to move demarcation point a little further, to some mini-PoP in the neighborhood, but to have choice the end-users need to own their lines there, and I'm not sure it's convenient and without the drawbacks.