Terrorist groups have shown a great capacity for organizational learning. Once a tactic is proven to be effective by one group, it spreads to others. This has been shown by the spread of suicide bombing, but also by other tactics like IEDs and beheadings, which started in the Middle East but have spread to the Mexican drug war. There is a great deal of concern about the possibility of WMD terrorism. While other techniques that have been proven to be effective have become widespread among various terrorist groups, WMD use is unlikely to become common place in the future due to the unique nature of WMDs.
Several terrorist groups, such as Al Qaeda, have tried to acquire WMDs, but few have been successful. Al Qaeda has released religious arguments for the use of a WMD, and has been trying to obtain a nuclear bomb for more than a decade. Some groups have managed to carry out WMD attacks, including the Aum Shinrikyo sarin attack on the Tokyo Subway. The main reason that it is unlikely that WMD attacks will spread is the high threshold involved in obtaining and using a WMD. Aum Shinrikyo was a unique case in that it was a religious cult that was allowed to operate a factory in Japan, had well educated scientists and was able to spend $30 million on its chemical weapon program. Despite all of these advantages, the sarin attack only killed 12 people. As long as WMDs remain hard to obtain and use, and conventional weapons remain easy and cheap, there is no reason to believe that WMD use will become widespread. If, in the future it becomes easier for terrorist groups to obtain or manufacture these weapons than the risk will become much higher.
Another difference between the use of a WMD and a tactic like suicide bombing is that the capability of WMDs already is fairly well known. Suicide bombing, when it was first used successfully, was a novel tactic, but WMDs have been used since at least WWI. Some terrorist groups may chose to not use a WMD even if they were able to. A mass casualty attack has the potential to turn the population against the group. WMDs are non-discriminating, they would kill a large number of sympathetic victims like children, so would only likely be used by groups with an ideology that could legitimize killing on that massive scale. Also, if a group used a WMD, there is a very high chance that governments and law enforcement would use overwhelming force to strike back, as seen as the American military response against Al Qaeda after the 9/11 attacks. The problem with WMD mass casualty attacks is that even one attack has the potential to be devastating, so it does not need to become a widespread trend to be a major threat that has to be watched and carefully prepared for.
 Pam Benson, “Official: Terrorists seek nuclear material, but lack ability to use it,” CNN, April 13, 2010. http://www.cnn.com/2010/US/04/13/nuclear.terrorists/
 Vernon Loeb, “Making Chemical Weapons is no easy task,” Washington Post, February 5, 2001. http://www.mvrd.org/avn/sonnie/news/05feb01_wp.htm