My simple objection is that mathematics training alone isn't enough to train a macroeconomist. Learning new tools will not change their ideas about economics. They will just use the tools to push their priors...
UPDATE: Hoisted from the comments.
«They will just use the tools to push their priors...»
But that is the whole purpose of Economics: confirming the central truthiness that the distribution of income is justified by productivity and creativity. The maths are used to give the respectability of formal methods to proofs of this central truthiness of Economics.