You said "I would think anything that helps bowel and colon health would be good for IBS though, especially since IBS involves the whole body not just one specific part."
Actually IBS involves a specific part of the body - the bowel - that is why it is called Irritable Bowel Syndrome. Its cause is believed to reside in the miscommunication between the brain in your head the brain in you gut - sometimes called a brain-gut dysfunction that may have a variety of origins. IBS may have secondary symptoms that effect other parts of the body or they may overlap with other disorders such as fibromyalgia but it definitely originates in the bowel. You might want to review the information on IBS on the International Foundation for Functional Gastrointestinal Disorders website.
There is some evidence to suggest that not all probiotics good for IBS and some might in fact exacerbate GI disorders. They use to think the colon contained 500 different types of bacteria, however, gene analysis has shown it contains more than 1500 different bacteria. Which of these 1500 bacteria is involved in IBS is difficult to say at the moment. To say anything that helps with colon health will help IBS is a problematic and a bit of naive assumption. It will take a considerable amount of research to determine which of this vast number of different bacteria play a role in IBS. It is difficult if not impossible to make generalizations of that kind without good evidence. There is good research evidence suggesting that acidophilus is not one of bacteria that plays a significant role in managing IBS. However, it could be that when it is combined with other probiotics like it is in VSL#3 that collectively it may have synergistic action with the other bacteria in the mixture but this hypothesis is unproven.
You say "I think doing research to find out what helps would become a little messy don't ya think?" Actually I think that the research protocols for IBS research have been pretty well established over the past few decades especially since the Rome criteria for diagnosis have been established.
You said "I think before they do studies like that they should break them in to groups based on probable causes and symptoms." Oh they do - what I directed you to was a meta-analysis of dozens of individual probiotic clinical trials for IBS. If you looked at the details of the individual studies you would find they are broken down by symptoms and a variety of other factors. Unfortunately, since no one knows the cause of IBS it is impossible to say what it is. In fact, part of the reason for doing IBS research is to identify causes and develop better management and diagnostic methods.
-------------------- STABLE: ♂, IBS-D 50+ years - Science of IBS