This recent thread, http://forum.keypublishing.com/showthread.php?t=119501 neatly illustrates the downside to not having barriers.

I would think that flight safety, personal safety, and security, coupled with the fact that there doesn’t seem to be an awful lot of respect for property these days is the reason for barriers, I think my only gripe with them is that they are often positioned too tightly to the displayed aircraft.