Typically, I would advocate any use of social media at a conference in order to engage the attendees in the room However, there are plenty of reasons not to do it so let's discuss those today, too.
First, a quick description of what a Twitter Wall is: a screen setup in a session room that is running a live feed of twitter posts about the event. Most commonly, the hashtag for the conference is the filter that is used so that viewers are only seeing posts about the event they are attending.
What's good about Twitter Walls
Showing the conversation that is happening on twitter during a presentation can connect the people in the room who are tweeting. A presenter or facilitator fielding questions during the session can see what people are asking on the twitter feed and everyone in the audience can see the questions that other people have. This is one of the best reasons to have a twitter wall. If you have a virtual audience that is viewing the same session, it connects them to the live audience. Someone in the room must be a virtual emcee or audience liaison for these folks.
If you have multiple break-out sessions happening simultaneously, your twitter users (and some non-twitter users) might appreciate being able to see tweets from other sessions. If your attendees are sharing educational tweets for the most part, then the audience will see all of the tweetable quotes coming from speakers in every room.
What's bad about Twitter Walls
If one of your attendees is having a bad experience, they may choose to share that information on the twitter feed. They will be expecting a response - and in most cases, the others on twitter will expect the organizer to respond as well. If the negative comment is directed at the speaker who is currently speaking, this can cause an embarrassing situation - whether or not the speaker is aware of it.
I recently spoke on a panel with someone who works for PETA. He told me the story of how their activists had taken over the twitter wall at a conference where someone from NASA was speaking. PETA was protesting something that NASA was doing at the time and bombarded the twitter hashtag at the conference without even being there. In this case, I don't think you can handle it by simply blocking negative tweeters. You would need to have someone that is monitoring the feed and have the ability to switch the feed to a different hashtag or put something else on the screen.
Another demographic to consider is your audience who is not on twitter. They may feel distracted by the twitter feed if they are not accustomed to viewing a twitter feed and listening to a speaker at the same time. As I have mentioned on other posts, if your attendees are using the hashtag to socialize more than they are using it to share knowledge, then the information showing may not be relevant to the physical audience. These folks will be turned off by twitter even more than they already are if your audience has become very elitist on twitter instead of welcoming.
Let me know what your experience has been with twitter walls at conferences. Do you have any pro's or con's that I missed?