Use case: raven.js takes over the global error handler as soon as it loads (316). It needs mw.track to not miss exceptions that happen before/during loading. It needs to ignore mw.track once it has loaded to avoid double reporting.
This is easy to work around with a flag in the subscribe callback so feel free to decline if you don't feel this capability would be useful in general. Otherwise, assign back to me and I'll write the patch.