The short answer is that the treaty didn't work very well. The tax haven countries were encouraged to sign bilateral treaties with other nations, and they went ahead and signed 300 or so of these treaties. But not every tax haven has a treaty with every country, and so the overall effect has been a relocation of money between tax havens. Here's the data they have available:
"For the purpose of our study, the Bank for International Settlements (BIS) has given us access to bilateral bank deposit data for 13 major tax havens, including Switzerland, Luxembourg, and the Cayman Islands. We thus observe the value of the deposits held by French residents in Switzerland, by German residents in Luxembourg, by US residents in the Cayman Islands and so forth, on a quarterly basis from the end of 2003 to the middle of 2011."The full list of the 13 tax havens is Austria, Belgium, the Cayman Islands, Chile, Cyprus, Guernsey, the Isle of Man, Jersey, Luxembourg, Macao, Malaysia, Panama, and Switzerland. These 13 jurisdictions account for about 75% of all the deposits of tax haven countries that report to the Bank of International Settlements. The authors also have data grouped together for five other tax havens: Bahamas, Bahrain, Hong Kong, the Netherlands Antilles, and Singapore. They write:
"We obtain two main results. First, treaties have had a statistically significant but quite modest impact on bank deposits in tax havens: a treaty between say France and Switzerland causes an approximately 11 percent decline in the Swiss deposits held by French residents. Second, and more importantly, the treaties signed by tax havens have not triggered significant repatriations of funds, but rather a relocation of deposits between tax havens. We observe this pattern in the aggregate data: the global value of deposits in havens remains the same two years after the start of the crackdown, but the havens that have signed many treaties have lost deposits at the expense of those that have signed few. We also observe this pattern in the bilateral panel regressions: after say France and Switzerland sign a treaty, French deposits increase in havens that have no treaty with France."
As with most studies, there are complications of interpretation. Are front companies being used to hide the movement of funds in a way that doesn't show up in these statistics? Perhaps as a response to the treaties some people are reporting to domestic tax authorities more of the income held in tax havens? This data set doesn't allow one to address that question. But the evidence from this study strongly suggests that trying to deal with tax havens through bilateral agreements is likely to be a very long-running game, and is ultimately unlikely to make much difference to how companies and individuals in the rest of the world are able to make use of tax havens.
Finally, James Hines wrote "Treasure Islands" for the Fall 2010 Journal of Economic Perspectives, which makes an effort to look at both the concerns over tax havens and some possible benefits they might convey. From the abstract: "The United States and other higher-tax countries frequently express concerns over how tax havens may affect their economies. Do they erode domestic tax collections; attract economic activity away from higher-tax countries; facilitate criminal activities; or reduce the transparency of financial accounts and so impede the smooth operation and regulation of legal and financial systems around the world? Do they contribute to excessive international tax competition? These concerns are plausible, albeit often founded on anecdotal rather than systematic evidence. Yet tax haven policies may also benefit other economies and even facilitate the effective operation of the tax systems of other countries."
Full disclosure: The AEJ:EP is published by the American Economic Association, which also publishes the Journal of Economic Perspectives, where I work as Managing Editor. All JEP articles, like the Hines article mentioned above, are freely available courtesy of the AEA.