I absolutely think this is a must.
In answers that I get notified about it happened quite often, that the questioner accepted all of the answers (probably not knowing, that this would de-accpet the previous one). Oftentimes people post similar answers, but with different links to helpful resources or similar, so it's not always possible to mark one answer as the right one.

As posted by Joseph on behalf of Norman A. above, allowing the marking of more than one answer as correct is important for user karma and points. But beyond that, it has value because two or more answers can contain nuances that could make: 1.) the choosing of just one as correct difficult for the questioner and 2.) if more than one answer has elements that make them both as 'go to' answers by later readers of the Q&A, it would be optimal to have them highlighted as such with the 'accepted' mark.

Another example: https://answers.atlassian.com/questions/12167884/answers/12167970/comments/12263552?

You can have answers that differ between the cloud and server versions. As far as I know Answers is suppose to be community driven without very active moderators to clean things up. Having questions/answers split based upon product the community would normally would not do it.

You can have answers that differ because the platforms they are using like Linux vs Windows or across the different databases.

It has been up to the person asking the question to choose his answers. If two people helped him because they assisted in different aspects of problem, he should be able to award those users.

Feedback from Norman A.

I think multiple answers is absolute necessity for user karma points and Answers. I think the comparison to stackoverflow is flawed since answers has different goals.
See question https://answers.atlassian.com/questions/9876324/is-it-possible-to-create-issues-for-service-desk-customers
as a good example of multiple answers..