As I documented here:
The extension-loading mechanism in maven 2.x is horrible, and prone to all sorts of weird behavioral changes depending on things like the number of artifacts in the extension's transitive dependency closure. IMO it needs to be replaced with something sensible. If we don't take the time to fix this mess, it's just going to extend the confusion surrounding extensions (which are problematic for a lot of users I've talked to) into the realm of toolchains as well.
I'm definitely -1 for patching the extension-loading code to support toolchains the same way wagons are supported now.