There's a nice summary of the Tax Justice Network and its campaign against tax havens over at the Land Value Taxation Campaign.
If somebody's objection to tax havens is that they enable the avoidance of tax through loopholes in the tax system, a brief period of consideration should make it obvious that the problem isn't tax havens, it's the tax system. Start levying tax on geographically fixed resources, such as locations, EM spectrum, roads, etc. and you solve the problem, because you can't move those items to a tax haven. Avoidance would be halted without any need to dictate tax policy to other nations.
The TJN appears unwilling to acknowledge that as an option, which seems to hint that their real objective is to establish a global tax cartel to ensure that taxes can be kept high everywhere without the fear of a more efficient competitor undercutting the agreed minimum.