Answers are still useful even if not being the one approved by the poster, as the reader can decide which one to go with. And anyone can vote on answers. Quite often a better answer will get more upvotes than the approved one, and readers (should) definitely skim for that. Sometimes the best piece of info is in a comment, and even that is quite OK visible as long as people vote for it.

The karma system also rewards upvotes more than approved answer in practice, since just 3 votes will be more karma than approved answer.