Right now the reading of toolchains is triggered by the maven-toolchains-plugin. This is not efficient, since the same files will be read over and over again in a multimodule project. This might be even be more tricky when building with multiple threads.
We probably can assume that if someone has specified a toolchains.xml, he also wants to use it. I don't see any reason why to wait until the first the maven-toolchains-plugin is called.