Copyblogger recently used the feature to gather some great responses from journalists and book authors for this post.
One issue with LinkedIn is that your question is published to search engines, and thus will be part of your search results forever. Some journalists are already wary of using ProfNet at times for the fear other journos will see what they’re working on and beat them to it. The even more public forum of LinkedIn can add to this concern.
Also, unlike ProfNet, LinkedIn Answers is not just for media queries. Meanwhile, other PR pros are taking things into their own hands.