Actually, it depends on the situation.
Let's see. You invite friend to a restaurant and order only a ceasar salad for yourself, while you friend takes a soup, a salad, some hot dish and a dessert. If you share the bill, you will pay more than you ate and your friend will pay less than he ate. It's good situation for your friend, but not for you.
Or if you're on a date, it's likely a boy will pay all the bill (if he's a nice boy). The bill can be shared in this situation though, but only in case if a girl ate a little more than a boy (once again -if a boy eats more than girl and they share their bill, it's unefficient for the girl).
In which situations a bill can be shared? Good question. Maybe when there is a huge company of friends and they are just spending time together, why not to share the bill instead of counting how much everybody should pay for themselves.
If there are only two of you going to a restaurant you can pay for the dishes you ate and it will be ok.
Conclusion: there are situations where it will be efficient to share the bill, but still - sometimes sharing a bill will take from you more money than you have to pay.