Very true, it is quite possible the effect may be placebo... afterall the IBS group apparently is one of the top groups to respond positively to placebo, indicating the mind may have a large role in controlling the spasms in our gut.
But what's interesting is that AA was used in a study of pigs deliberately infected with e.coli - all pigs had diahorrea. Those treated with AA had their diahorrea almost completely eliminated whereas acupuncture was only helpful in pigs which had been recently infected with e.coli. The results from this study were that, AA via moxibustion was effective in chronic patients, and acupuncture was effective in early onset but ineffective in chronic patients. This ironically was reported many centuries ago by a famous chinese herbalist who suggested moxi for chronic and more persistant conditions.
More importantly, there has recently been many high profile studies around sweet wormwood (of the same family as AA).http://www.newmediaexplorer.org/sepp/2004/07/17/sweet_wormwood_heals_malaria.htm
The ingredients of sweet wormwood have been extracted to create a very very potent and effective drug against malaria. Even the gates foundation - who are committed to helping those in africa - have pledged money for ongoing research into the sweet wormwood drug. It is so potent that the effects of malaria are going within 24 hours in most cases... malaria has been one of the biggest killers in africa by the way.
Considering most of our western drugs today come from plants anyhow, there definately is a possibility that there is something other than a placebo effect happening here. Researchers just need financial support and will to turn these plants into usable drugs.